Add basic documentation

README.md

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+# Apheleia
+
+Good code is automatically formatted by tools like
+[Black](https://github.com/python/black) or
+[Prettier](https://prettier.io/) so that you and your team spend less
+time on formatting and more time on building features. It's best if
+your editor can run code formatters each time you save a file, so that
+you don't have to look at badly formatted code or get surprised when
+things change just before you commit. However, running a code
+formatter on save suffers from the following two problems:
+
+1. It takes some time (e.g. around 200ms for Black on an empty file),
+   which makes the editor feel less responsive.
+2. It invariably moves your cursor (point) somewhere unexpected if the
+   changes made by the code formatter are too close to point's
+   position.
+
+Apheleia is an [Emacs](https://www.gnu.org/software/emacs/) package
+which solves both of these problems comprehensively for all languages,
+allowing you to say goodbye to language-specific packages such as
+[Blacken](https://github.com/proofit404/blacken) and
+[prettier-js](https://github.com/prettier/prettier-emacs).
+
+The approach is as follows:
+
+1. Run code formatters on `after-save-hook`, rather than
+   `before-save-hook`, and do so asynchronously. Once the formatter
+   has finished running, check if the buffer has been modified since
+   it started; only apply the changes if not.
+2. After running the code formatter, generate an [RCS
+   patch](https://tools.ietf.org/doc/tcllib/html/rcs.html#section4)
+   showing the changes and then apply it to the buffer. This prevents
+   changes elsewhere in the buffer from moving point. If a patch
+   region happens to include point, then use a [dynamic programming
+   algorithm for string
+   alignment](https://en.wikipedia.org/wiki/Needleman%E2%80%93Wunsch_algorithm)
+   to determine where point should be moved so that it remains in the
+   same place relative to its surroundings. Finally, if the vertical
+   position of point relative to the window has changed, adjust the
+   scroll position to maintain maximum visual continuity. (This
+   includes iterating through all windows displaying the buffer, if
+   there are more than one.) The dynamic programming algorithm runs in
+   quadratic time, which is why it is only applied if necessary and to
+   a single patch region.
+
+## User guide
+
+To get started with Apheleia, install it with
+[`straight.el`](https://github.com/raxod502/straight.el) as follows:
+
+    (straight-use-package '(apheleia :host github :repo "raxod502/apheleia"))
+
+Alternatively, you can use another source-based package manager such
+as [Quelpa](https://framagit.org/steckerhalter/quelpa),
+[El-Get](https://github.com/dimitri/el-get), or
+[Borg](https://github.com/emacscollective/borg). Apheleia is not
+currently listed on [MELPA](https://melpa.org/) or [GNU
+ELPA](https://elpa.gnu.org/).
+
+To your init-file, add the following form:
+
+    (apheleia-global-mode +1)
+
+The autoloading has been configured so that this will *not* cause
+Apheleia to be loaded until you save a file.
+
+By default, Apheleia is configured to format with
+[Black](https://github.com/python/black),
+[Prettier](https://prettier.io/), and
+[Gofmt](https://golang.org/cmd/gofmt/) on save in all relevant major
+modes. To configure this, you can adjust the values of the following
+variables:
+
+* `apheleia-formatters`: Alist mapping names of formatters (symbols
+  like `black` and `prettier`) to commands used to run those
+  formatters (such as `("black" "-")` and `(npx "prettier" input)`).
+  See the docstring for more information.
+* `apheleia-mode-alist`: Alist mapping major modes and filename
+  regexps to names of formatters to use in those modes and files. See
+  the docstring for more information.
+* `apheleia-formatter`: Optional buffer-local variable specifying the
+  formatter to use in this buffer. Overrides `apheleia-mode-alist`.
+
+You can run `M-x apheleia-mode` to toggle automatic formatting on save
+in a single buffer, or `M-x apheleia-global-mode` to toggle the
+default setting for all buffers. Also, even if `apheleia-mode` is not
+enabled, you can run `M-x apheleia-format-buffer` to manually invoke
+the configured formatter for the current buffer. Running with a prefix
+argument will cause the command to prompt you for which formatter to
+run.
+
+## Contributing
+
+Development of Apheleia happens using the provided Makefile:
+
+    % make help
+    usage:
+      make all        Build project and run all linters
+      make compile    Check for byte-compiler errors
+      make checkdoc   Check for missing or poorly formatted docstrings
+      make longlines  Check for lines longer than 79 characters
+      make clean      Remove build artifacts
+      make docker     Start a Docker shell; e.g. make docker VERSION=25.3
+      make help       Show this message
+
+All commits are automatically tested using `make all` (== `make`) for
+all supported Emacs versions on the excellent [Semaphore
+CI](https://semaphoreci.com/) platform. Please make sure you can
+successfully run `make` before submitting a pull request.
+
+If the CI fails, it may be that your change is not compatible with one
+of the Emacs versions supported by Apheleia. Suppose that the failure
+is for Emacs 25.3. To test this locally, you can install
+[Docker](https://www.docker.com/) and run:
+
+    % make docker VERSION=25.3
+
+This will start a shell inside of Docker with the specified version of
+Emacs available. The Apheleia source code on your filesystem is
+synchronized with the filesystem inside of Docker, so you need not
+restart the shell after making changes. To test, simply use the same
+Makefile targets as usual.
+
+## Acknowledgements
+
+I got the idea for using RCS patches to avoid moving point too much
+from [prettier-js](https://github.com/prettier/prettier-emacs),
+although that package does not implement the dynamic programming
+algorithm which Apheleia uses to guarantee stability of point even
+within a formatted region.
+
+Note that despite this inspiration, Apheleia is a clean-room
+implementation which is free of the copyright terms of prettier-js.