The cc65 docs say VICE labels start with '.'. Also, output local
labels prefixed with '@', per ca65 convention. The default output
file is now "labels.lbl".
Added some minimal documentation.
(issue #151)
Move the SourceGen manual to a subdirectory in "docs", so that it can
be accessed directly from the 6502bench web site. The place where
it's installed in the distribution doesn't change.