Add ISSUE_TEMPLATE.md

.github/ISSUE_TEMPLATE.md

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+* gevent version:
+* Python version:
+* Operating System:
+
+### Description:
+
+```
+// REPLACE ME: What are you trying to get done, what has happened, what went wrong, and what did you expect?
+```
+
+### What I've run:
+
+```
+// REPLACE ME: Paste short, self contained, correct example code (sscce.org), tracebacks, etc, here
+```