Update CONTRIBUTING.rst

Add not about how to use the issue tracker

CONTRIBUTING.rst

@@ -3,6 +3,11 @@ Guidelines for Contributing to github3.py
 #. Read the README_
+#. Please do **not** use the issue tracker for questions.
+
+#. Please use GitHub's search feature to look for already reported issues 
+   before reporting your own.
+
 #. Regardless of the magnitude your pull request (a couple lines to a couple 
    hundred lines), please add your name to the AUTHORS.rst_ file under the 
    heading Contributors.