docs: add contributing guideline

docs: add issue and PR templates

.github/CONTRIBUTING.md

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+# How-to contribute
+
+Those are the main contributing guidelines for contributing to this project:
+
+- Verify that your contribution does not embark proprietary code or infringe any copyright of any sort.
+- Avoid adding any unnecessary dependencies to the project, espcially of those are not easily packaged and installed through `conda` or `pip`.
+- Python contributions must follow the [PEP 8 style guide](https://www.python.org/dev/peps/pep-0008/).
+- Use [Pull Request](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests) mechanism and please be patient while waiting for reviews.
+- Remain polite and civil in all exchanges with the maintainers and other contributors.

.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..

.github/PULL_REQUEST_TEMPLATE.md

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+# [Spleeter-XXXX] - pull request title
+
+## Description
+
+A few sentences describing the overall goals of the pull request's commits.
+
+## How this patch was tested
+
+You tested it, right?
+
+## Documentation link and external references
+
+Please provide any info that may help us better understand your code.