Add mention of version_slug parameter when using webhook (#2599)

Signed-off-by: Thanh Ha <[email protected]>

Fixes #2138

Author: zxiiro

docs/webhooks.rst

@@ -48,6 +48,14 @@ look something along the lines of ``http://readthedocs.org/build/<project_name>`
 Regardless of which revision control system you use, you can just hit this URL
 to kick off a rebuild.
 
+The following parameters available to customize the behavior of custom webhooks:
+
+* ``'version_slug'``: The build version to trigger build for (defaults to ``'latest'``)
+
+  Example::
+  
+      $ curl -X POST --data "version_slug=$VERSION" https://readthedocs.org/build/$PROJECT_NAME
+
 You could make this part of a hook using Git_, Subversion_, Mercurial_, or
 Bazaar_, perhaps through a simple script that accesses the build URL using
 ``wget`` or ``curl``.