add donwload info and examples

README.md

@@ -9,6 +9,14 @@ This extension injects KaTeX into the `Riot` chat client for
  - `$$x^2$$` renders math block style
  - `\(x^2\)` renders math inline
 
+## Example
+[Example](example.png)
+
+## Download and Installation
+You can download the extension [from
+here](https://github.com/vale981/riot-katex/releases/latest). Just
+double click it, or drag it one firefox to install.
+
 ## Building the Extension
  - make sure you have npm installed
  - run `./package.sh`, the webextension will be zipped as