docs: add linkable section about updating commit messages in `CONTRIBUTING.md` (#39215)
docs: add linkable section about updating commit messages in `CONTRIBUTING.md` (#39215)

A common review request is updating the commit message of a commit.
Since this is something that is not straight forward for inexperienced
contributors, it is useful to be able to point a contributor to some
docs outlining the process.

This commit adds such a section in `CONTRIBUTING.md` (as discussed in
https://github.com/angular/angular/pull/39110#discussion_r499935502).

PR Close #39215

brandonroberts/angular,shairez/angular,angular/angular,manekinekko/angular,ocombe/angular,angular-indonesia/angular,gkalpak/angular,alxhub/angular,ocombe/angular,gnomeontherun/angular,mgechev/angular,mgechev/angular,brandonroberts/angular,manekinekko/angular,mgechev/angular,manekinekko/angular,ocombe/angular,JiaLiPassion/angular,gnomeontherun/angular,gnomeontherun/angular,brandonroberts/angular,mgechev/angular,manekinekko/angular,JiaLiPassion/angular,blesh/angular,ocombe/angular,gkalpak/angular,angular-indonesia/angular,mgechev/angular,IdeaBlade/angular,IdeaBlade/angular,shairez/angular,angular/angular,IdeaBlade/angular,alxhub/angular,blesh/angular,brandonroberts/angular,angular/angular,gnomeontherun/angular,shairez/angular,angular/angular,shairez/angular,blesh/angular,brandonroberts/angular,manekinekko/angular,shairez/angular,ocombe/angular,ocombe/angular,brandonroberts/angular,alxhub/angular,gkalpak/angular,blesh/angular,alxhub/angular,blesh/angular,angular-indonesia/angular,manekinekko/angular,alxhub/angular,IdeaBlade/angular,IdeaBlade/angular,angular-indonesia/angular,JiaLiPassion/angular,gkalpak/angular,JiaLiPassion/angular