Create CONTRIBUTING.md

[bsclifton]

Fixes #108

Easy to read version located here:
https://github.com/brave/brave-browser/blob/bsc-add-contributing-md/CONTRIBUTING.md

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Requested a security/privacy review as needed.
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protection-Mode
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection

Test Plan:

N/A

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions.

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[bsclifton]

Still a WIP - but feedback very welcome!

[pes10k]

For my two cents, the main things I think that'd be useful to have in a guide (thats not already in the guide) is how to update, apply and rebase the patch set

[jumde]

More:

1. When/How to patch (#defines in chromium_src)
2. Subclassing (in chromium_src) vs Patching

[bsclifton]

OK - I think this is finally ready for review! 😄

edit:
Good call, @jumde - will have to add those

[bsclifton]

Comments by @cezaraugusto addressed - ready for re-review 😄