Remove reference to .relative_gitmodules from README.txt

Quick fix to remove reference to .relative_submodules from README.txt, which is a bit confusing since we're no longer doing it that way.

By the way, the instructions also say to build by just running "make", which doesn't work for me (I'm on Linux).  I have to run "make -f Makefile", otherwise the build crashes out with lots of errors, the first of which is:

src/Core/Debug/OOJSConsole.h:33:19: warning: jsapi.h: No such file or directory

Are the instructions wrong, or is something broken somewhere else?