docs: add contributing guideline

docs: add issue and PR templates

.github/ISSUE_TEMPLATE.md

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+# ISSUE-TITLE
+
+## Describe the bug/issue
+
+First tell us:
+
+- which part of Spleeter is it about (Doc, Scripts, Docker, etc )?
+- what type of issue is it ? ( BUG / QUESTION / FEATURE REQUEST ?) ..
+
+Then give us a clear and concise description of the issue.
+
+## To reproduce
+
+Steps to reproduce the behavior :
+
+1. Install with '...'
+2. Run '....'
+3. See error
+
+## Terminal output
+
+Share what your terminal says when you run the script.
+
+## Your Setup
+
+In particular:
+
+- OS: [Windows/Linux/Mac/Other]
+- Execution Environment :[Conda/Pip/Other] and versions
+
+## Additional context
+
+Add any other context about the problem here, references, cites, etc..