DOC: add issue template

Author: eigenfoo

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,20 @@
+## Problem Description
+
+**Please provide a minimal, self-contained, and reproducible example:**
+```python
+[Paste code here]
+```
+
+**Please provide the full traceback:**
+```python
+[Paste traceback here]
+```
+
+**Please provide any additional information below:**
+
+
+## Versions
+
+* Pyfolio version:
+* Python version:
+* Matplotlib version: