doc: Tidy up first part of top-level README file

First (small) pass at tidying up the README file, including:

 * remove references to obsolete CREDITS file
 * remove (some) references to obsolete boards.cfg file
 * remove at least one reference to a "scrapped" board
 * cut down unnecessarily detailed directory hierarchy
 * bunch of grammar and spelling tweaks

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>