Add a Usage section explaining the vimrc

Author: jbranchaud

README.md

@@ -412,6 +412,12 @@ _340 TILs and counting..._
 - [Whole Line Auto-Completion](vim/whole-line-auto-completion.md)
 - [Wrap With Some Room](vim/wrap-with-some-room.md)
 
+## Usage
+
+The `.vimrc` file for this project contains a function `CountTILs` that can
+be invoked with `<leader>c`. This will do a substitution count of the
+current number of TILs and display the result in the command tray.
+
 ## About
 
 I shamelessly stole this idea from