add a link to the API docs in the README

Author: patrickdevivo

README.md

@@ -111,3 +111,7 @@ brew install tickgit
 ### Usage
 
 The most up to date usage will be the output of `tickgit --help`. The most common usage, however, is `tickgit status` which will print a status report of tickets for a given git repository. By default, it uses the current working directory.
+
+### API
+
+To find information about using the tickgit API, see [this file](https://github.com/augmentable-dev/tickgit/blob/master/docs/API.md).