Added API documentation

Author: mickem

docs/api/index.md

@@ -1,15 +1,6 @@
-.. _api-index:
+# NSClient++ API
 
-# API #
+This is a placeholder for where the real API docs will be.
 
-TODO
-
-Contents:
-
-<dl>
-  <dt>.. toctree :  : </dt>
-  <dd>
-    <p>:maxdepth: 1</p>
-    <p>plugin.rst</p>
-  </dd>
-</dl>
+Currently we have documentation for the [protocol buffer API used by plugins](plugin.md).
+This is very similar to the current REST API as well as it uses an json version of the same.