Create ISSUE_TEMPLATE.md

Add an issue template to help user report issues.

Author: Junpeng Lao

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,17 @@
+If you have questions about specific usecase, please post it to our discourse channel: https://discourse.pymc.io
+If you are not sure this is bug or not, please also post it on our discourse.
+
+## Description of your problem
+
+**Please provide a minimal, self-contained, and reproducible example.**
+
+**Please provide any additional information below**
+
+
+## Versions and main components
+
+* PyMC3 Version:
+* Theano Version:
+* Python Version:
+* Operating system:
+* How did you install PyMC3: (conda/pip)