Rewrite (was: fix) compile instructions

From the git-clone(1) manpage: "The 'humanish' part of the source repository is used **if no directory is explicitly given**[...]". Since we are explicitly giving it a directory, we need to give it the humanish part, too.

Also, the host triplet for mingw32 is "i686-pc-mingw32-".

Edit: And, you know, SDL instructions were missing. So I went and rewrote the whole document.