Add features section to README.

README.mkd

@@ -1,7 +1,22 @@
-## Vim Git Gutter
+## vim-gitgutter
 
 A Vim plugin which shows a git diff in the 'gutter' (sign column).  It shows whether each line has been added, modified, and where lines have been removed.  You can also stage and revert individual hunks.
 
+Features:
+
+* shows signs for added, modified, and removed lines
+* quick jumping between blocks of changed lines ("hunks")
+* stage/revert individual hunks
+* optional line highlighting
+* fully customisable (signs, sign column, line highlights, mappings, extra git-diff arguments, etc)
+* can be toggled on/off
+* preserves signs from other plugins
+* easy to integrate diff stats into status line; built-in integration with [vim-airline](https://github.com/bling/vim-airline/)
+
+Constraints:
+
+* supports git only
+
 
 ### Screenshot