Accessible "building from source" instructions

[Dmitry Gutov]

I've been looking for a simple instructions to point users to, so 
anybody can try building an Emacs 27 snapshot.

Here's the closest official URL I could find: 
https://www.gnu.org/software/emacs/CONTRIBUTE

The script at the top should be a good starting point, but it does more 
than just "build and run Emacs from scratch".

In particular, it changes the user's Git configuration. And if somebody 
just copy-pastes in into their terminal, it will bork their global Git 
config. So how about we remove those lines, or move them to a separate 
section?

Or is there a better page I can link to?