Add PR template & contributing doc

[cameri]

Description

1. Add PR template
2. Add CONTRIBUTING.md

Motivation and Context

1. Add documentation to incentivize contributions

Types of changes

• Non-functional change (docs, style, minor refactor)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my code changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 3301253880

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 64.737%

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Totals
Change from base Build 3300735365:	0.0%
Covered Lines:	425
Relevant Lines:	683

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💛 - Coveralls