Add Guides and Documentation

[n7studios]

Summary

Adds markdown documentation, including how to contribute to this SDK, including how to setup the development environment, development, testing and submitting a PR.

This PR separately covers documentation for developers wanting to use the PHP SDK in their application.

Adds issue templates with fields when submitting an issue.

For review, it may be easier to preview markdown files by previewing the branch itself

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)