Add basic Github Zuul Reporter.

Change-Id: I3c34bb1354adb7c360e173c227f00bf987b7d30e
Co-Authored-By: Jan Hruban <jan.hruban@gooddata.com>
diff --git a/doc/source/reporters.rst b/doc/source/reporters.rst
index b01c8d1..4972e66 100644
--- a/doc/source/reporters.rst
+++ b/doc/source/reporters.rst
@@ -28,6 +28,12 @@
 A :ref:`connection` that uses the gerrit driver must be supplied to the
 trigger.
 
+GitHub
+------
+
+Zuul reports back to GitHub pull requests via GitHub API.
+It will create a comment containing the job status.
+
 SMTP
 ----