Fixed typos.  And, fixed descriptions of where ld65 looks for files.
Merge pull request #110 from greg-king5/typo

Fixed typos; and, fixed descriptions of where ld65 looks for files.
Put mouse cursor shape definition into .rodata.
Document mouse callbacks and some other small changes.
Merge pull request #111 from groessler/something_to_pull

Document Atari mouse callbacks