Clarify team member contribution policy

[matthew-reynolds]

📖 Documentation Error

Our CONTRIBUTING.md is a bit confusing and doesn't do a good job of explaining that within-team members should contribute by creating branches within the main repo, as opposed to external contributors who should fork. Need to clarify this language.

[matthew-reynolds]

Elevating to urgent - This doesn't block any specific other issues or anything, but it's a fairly significant barrier to getting more collaborators involved. In particular, this must be done before our next recruiting round completes so that the new team members have a smoother experience joining the team.

[matthew-reynolds]

Other things to add to the guide:

1) No force pushing

2) Clarify how to commit. Commit messages must:

• Separate subject from body with a blank line
• Limit the subject line to 50 characters
• Capitalize the subject line
• Do not end the subject line with a period
• Use the imperative mood in the subject line
• Wrap the body at 72 characters
• Use the body to explain what and why vs. how

We should add that the body is optional, maybe change the imperative to "imperative or past-tense" since even though that's incorrect and inconsistent, it's more in line with how things are likely to be done. Idk I have mixed opinions on that last one.

Adding a couple examples of good commit messages might also be a good idea.

[wmmc88]

@michaelwm thoughts?