Oh. I forgot:
Befor editing you'll need to get rules(md).ini file. How do get it?
You'll need XCC Mixer (download here).
Then using XCC Mixer open Ra2.mix (if you edit Red Alert 2) or Ra2md.mix (Yuri's Revenge) and extract local.mix (from Ra2.mix) or localmd.mix (from Ra2md). Then open local(md).mix and extract rules(md).ini
P.S. All files in YR have the same names as in Ra2, but with addition 'md'
For beginners: hacking is cracking, modifying with no permission of authors. All things we edit are knowingly open, and this editing is legal. Such kind of editing is called modding.