INSTALL.md Out of Date

[stevedor]

The instructions in INSTALL.md appear to be out of date. In particular, they seem to rely on an old version of rgbds. Following the instructions and pulling the git branches listed there, I got the following error while running make in pokecrystal/:

pokecrystal$ make

rgbasm -o rgbdscheck.o rgbdscheck.asm
ERROR: rgbdscheck.asm(14):
    pokecrystal requires rgbds v0.6.0 or newer.
make: *** [Makefile:124: rgbdscheck.o] Error 1

However, it seems to work fine if you pull the current version of rgbds:

$ git clone --depth=1 https://github.com/rednex/rgbds

I then ran make -C against that directory, added it to the path manually, and both it and pokecrystal built fine on Ubuntu 21.04.1 LTS.

I'm not sure if this will work for everyone, though, and don't want to put in a PR without being able to check this myself.