document Atari mouse drivers and linker config files
rename 'builtin config file' to 'default config file'; remove
references to '--dump-config'; remove a superfluous word
Add better explanation how to leave out parts of the EXE file in the
atari-asm.cfg description.
Merge pull request #72 from groessler/gh-pages_to_pull

document Atari mouse drivers and linker config files