Step-by-step instructions to submit a PR

[dhmlau]

Description / Steps to reproduce / Feature proposal

There is information on how to submit a PR for LB4 / loopback-next repo. As @raymondfeng pointed out, it might be useful to have step-by-step instructions to make a PR, similar to https://www.nearform.com/blog/getting-into-open-source-for-the-first-time/.

From @raymondfeng:

Here are some key things to cover:
1. folk `loopback-next` repo
2. git clone the folked repo
3. make changes locally
4. Run `npm it` to make sure everything is working
5. commit changes
6. push commits to the folked repo
7. create a PR
8. Sign CLA
9. go through PR review process
10. make more changes and push back to the folked repo
11. Rebase against latest master@upstream
12. Squash commits to create clean history for the PR

Acceptance Criteria

• Create a new page that contains the instructions
  • Some existing info for reference: https://github.com/strongloop-internal/loopback-knowledge-base/blob/master/rebasing-community-prs.md
• Add a pointer at the LB4 contribution page: https://loopback.io/doc/en/contrib/code-contrib-lb4.html
• Verify that the steps are accurate by either having a new member contribute a PR and go through the outlined steps or us simulating a new user going through the contribution process.

cc @raymondfeng @emonddr

[emonddr]

@raymondfeng and I had a meeting to discuss the purpose, content and layout of this document.

1. There should be a set of instructions for github experts to follow that mainly focuses on things specific to the loopback.

Some things to mention :

• signing the CLA
• how to amend commit messages
• fixing formatting issues and running : npm run test
• making sure the CI jobs complete successfully
• the review process : handling multiple commits, rebasing, squashing
• that the PR template Checklist is the pre-condition for PR approval
• that the successful run of the CI jobs is the post-condition for PR approval

2. There should also be a set of instructions for github beginners to follow that takes them through the github steps, and the things specific to loopback.