Update CONTRIBUTING.md to add a contribution guideline

[CodeMaster7000]

This is a follow-up to issue #236 and creates a common, friendly ground so people can come to terms with how to commit to this repository.

[vercel]

@CodeMaster7000 is attempting to deploy a commit to the ryo-ma's Team Team on Vercel.

A member of the Team first needs to authorize it.

[ryo-ma]

LGTM

[ryo-ma]

Can you check the CI? Please fmt.
https://github.com/ryo-ma/github-profile-trophy/actions/runs/6422379016/job/17438742963?pr=238

[CodeMaster7000]

@ryo-ma There appears to be a formatting issue.

[ryo-ma]

yes, I know that.
you should run the deno fmt

[CodeMaster7000]

@ryo-ma I have and it gives some additions and deletions - I'm not sure what to do.

[ryo-ma]

Check the https://github.com/ryo-ma/github-profile-trophy/blob/master/CONTRIBUTING.md#what-to-do-before-contributing.

And can you rebase? This PR conflicted

[ryo-ma]

I probably can't because this branch is your forked repository.

Did you rebase? If you don't understand that, you should study Github and Git thoroughly.

[ryo-ma]

I'm not in a hurry so I'll wait.
I'm not going to commit it to your repository.

If you do not fix it, I will close this PR and commit directly to my master branch. This method is the least labor-intensive for me.

[CodeMaster7000]

@ryo-ma I believe the conflicts have been resolved now. I will now address the fmt. I am new to this so please request changes if I made an error somewhere.

Thank you so much for your patience.

[ryo-ma]

LGTM