DEV: add issue template

.github/ISSUE_TEMPLATE.md

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+Dear Zipline Maintainers,
+
+Before I tell you about my issue, let me describe my environment:
+
+# Environment
+
+* Operating System: (Windows Version or `$ uname --all`)
+* Python Version: `$ python --version`
+* Python Bitness: `$ python -c 'import math, sys;print(int(math.log(sys.maxsize, 2) + 1))'`
+* How did you install Zipline: (`pip`, `conda`, or `other (please explain)`)
+* Python packages: `$ pip freeze` or `$ conda list`
+
+Now that you know a little about me, let me tell you about the issue I am
+having:
+
+# Description of Issue
+
+* What did you expect to happen?
+* What happened instead?
+
+Here is how you can reproduce this issue on your machine:
+
+## Reproduction Steps
+
+1.
+2.
+3.
+...
+
+## What steps have you taken to resolve this already?
+
+...
+
+# Anything else?
+
+...
+
+Sincerely,
+`$ whoami`