Update issue templates

.github/ISSUE_TEMPLATE/feature-requests.md

+---
+name: Feature Requests
+about: Suggest an idea for this project
+
+---
+
++ 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.

.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.

.github/ISSUE_TEMPLATE/unexpected-problems---bugs.md

+---
+name: Unexpected Problems / Bugs
+about: For use with unexpected problems of Tensorpack.
+
+---
+
+__PLEASE ALWAYS INCLUDE__:
+1. What you did:
+  + If you're using examples:
+    + What's the command you run:
+    + Have you made any changes to code? Paste them if any:
+  + If not, tell us what you did that may be relevant.
+    But we may not investigate it if there is no reproducible code.
+  + Better to paste what you did instead of describing them.
+2. What you observed, including but not limited to the __entire__ logs.
+  + Better to paste what you observed instead of describing them.
+3. What you expected, if not obvious.
+4. Your environment:
+  + Python version.
+  + TF version: `python -c 'import tensorflow as tf; print(tf.GIT_VERSION, tf.VERSION)'`.
+  + Tensorpack version: `python -c 'import tensorpack; print(tensorpack.__version__)'`.
+      You can install Tensorpack master by `pip install -U git+https://github.com/ppwwyyxx/tensorpack.git`.:
+  + Hardware information, if relevant.
+
+About efficiency issues, PLEASE first read http://tensorpack.readthedocs.io/en/latest/tutorial/performance-tuning.html

.github/ISSUE_TEMPLATE/using-understanding-tensorpack.md

+---
+name: Using/Understanding Tensorpack
+about: More general questions about Tensorpack.
+
+---
+
++ Read the [tutorials](http://tensorpack.readthedocs.io/en/latest/tutorial/index.html#user-tutorials) first.
++ We answer "HOW to do X with Tensorpack" for a well-defined X.
+  We also answer "HOW/WHY Tensorpack does X" for some X that Tensorpack or its examples are doing.
+
+  We __don't__ answer general machine learning questions, such as "why my training doesn't converge", "what networks to use" or "I don't understand the paper".
+
+You can also use gitter (https://gitter.im/tensorpack/users) for more casual discussions.