Update issue templates

.github/ISSUE_TEMPLATE/feature_request.md

----
-name: Feature request
-about: Suggest an idea for this project
-
----
-
-Note that:
-
-+ You can implement a lot of features by extending Tensorpack
-  (See http://tensorpack.readthedocs.io/en/latest/tutorial/index.html#extend-tensorpack).
-  It does not have to be added to Tensorpack unless you have a good reason.
-+ "Could you improve/implement an example/paper ?" 
-  -- The answer is: we have no plans to do so. We don't take feature requests for
-  examples or implement a paper for you. If you don't know how to do it, you may ask a usage question.