Readme is tremendously out of date

The readme should be rewritten because a lot of the information it contains is no longer relevant.

Preferably, the readme should be written as a Markdown document so that it's both plaintext readable and formatted correctly in GitLab.

It should include instructions for building with CMake for each major platform, the GPLv2 header, and a link to the wiki page for compiling (for the old Makefile system).