Readme is tremendously out of date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).

mentioned in commit 26744c2a

Status changed to closed

Did pretty much all of this here: http://git.magicalgirl.moe/STJr/SRB2/merge_requests/111

mentioned in issue #21

mentioned in commit 5ec24756

mentioned in commit 0e5bfdf0

mentioned in merge request !389 (merged)

mentioned in merge request !1444 (merged)