Update README.md
Merge commit '81398e7dfae3f153e2950435e2c9812b64848d9a' into HEAD
Use Verdana font for web pages.  Updated some .gitignore files.  Removed obsolete unversioned-files.txt and unversioned-files-extra.txt files (which were for svn).  Added some error-checking for proof-checker S and SL commands.  Cause error when acl2-doc download attempt creates 0-length file.  Eliminated deprecated sharp-comma-reader.

This comment added to (defxdoc note-7-0 ...) in
books/system/doc/acl2-doc.lisp explains the proof-checker
error-checking mentioned above.

; It had been possible to get a raw Lisp error when invoking the proof-checker
; S or SL command with a non-numeric backchain-limit.  Here is an example.
;
;   (defthm foo
;     (implies (consp x) (< 0 (len x)))
;     :rule-classes ((:rewrite :backchain-limit-lst 0)))
;   (verify (implies (and (character-listp x) x)
;                    (characterp (nth 0 x))))
;   ; Each of these caused a raw Lisp error, but now cause a graceful ACL2
;   ; error:
;   (sl abc)
;   (s :backchain-limit abc)
;
; Error-checking has also been added for the :repeat argument of S, though we
; haven't seen a raw Lisp error for a bad :repeat argument.

Quoting :doc note-7-0:

  The reader macro sharp-comma #,, which has been deprecated since ACL2
  Version 3.5 (May, 2009), has been eliminated. Use sharp-dot #.
  instead; see [sharp-dot-reader].
Synched doc.lisp and tweaked new :doc note-7-0 item about the elimination of sharp-comma-reader.
Added .gitignore files to clean up output from "git status".