Improve README.md and focus on relevant content

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Additionally, a bunch of instructions were outdated / no longer relevant, updated those.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.
Merge pull request #1430 from akoeplinger/readme

Improve README.md and focus on relevant content