Add information about recursing submodules for markup2 to the readme

[randomouscrap98]

Literally just the title, make sure people know to pull the submodules

[randomouscrap98]

In case I forget, the command for downloading them after the fact is:

git submodule update --init --recursive

and to download them on clone:

git clone --recursive

[ilovecherries]

There is no longer a need to add this to the documentation since it is now included as a package.