Before, the first thing the user saw were instructions for niche platforms like Itanium or Solaris.
Instead, we now just link to the "Compiling Mono" guides for OSX, Linux and Windows on the Mono website.

Additionally, a bunch of instructions were outdated / no longer relevant, updated those.

The "Configuration options" section was moved to the bottom of the readme, it was pretty long and pushed
the other sections into oblivion.

Add info on parallel installations which is going to be the case for 99% of people trying to compile and use mono from source.

New section has links to guideline wiki page, and the page's Important Rules section that all contributors should be aware of.