Contrib. Guide: Add recent advice related to git commits

[balacij]

          @balacij you provide some great advice [above](https://github.com/JacquesCarette/Drasil/pull/3705#issuecomment-2148869719).  Can you or one of the summer students add this advice to our Contributor's Guide?  I like your advice about squashing commits and only merging with master when necessary.  Your advice on commit messages is also valuable.  We might already have some of this in the Contributor's guide.

Originally posted by @smiths in #3705 (comment)

Don't want to lose this, but I know that if I start this now, I will be tempted to review a bit more too.

[smiths]

That is a good use of the issue tracker @balacij.

[daijingz]

I think, There are two parts: title and content
The title summarizes contributions, with no more than 6 words.
While the content uses bullet points to indicate ongoing updates