rn-54: add an introduction to git-filter-repo

[newren]

This introduction focuses on two things:

• Providing a simple example to give a flavor for git-filter-repo usage
• Compares to existing tools and explains why I created a new tool instead of extending others

But it also provides several links for folks that want to know more.

[chriscool]

Thanks a lot @newren ! I will merge this. Feel free to make further improvements later though.

[chriscool]

@newren I moved your article to the top, added (written by Elijah Newren) near its title and mentioned you among the helpers at the bottom of the edition.

[newren]

Awesome, thanks!