add issue template

.github/ISSUE_TEMPLATE.md

+Bug Reports/Feature Requests/Usage Questions Only:
+
+Bug Reports:
+Some part of code (either the library or examples) doesn't work as expected.
+
+Feature Requests:
+1. Improve an existing feature.
+2. Add a new feature. Please 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 may not have to be added to tensorpack unless you have a good reason.
+3. Note that we don't take "example requests".
+
+Usage Questions:
+Usage questions are like "How do I do [this specific thing] in tensorpack?".
+We don't answer general machine learning questions like:
+"I want to do [this machine learning task]. What specific things I need to do?"