Update README.rst

Update the README to elaborate on case-sensitivity in ignored words
and dictionaries.

Add an additional "Ignoring Words" section, with internal link from the
"noteworthy flags" overview.

Co-authored-by: Peter Newman <peternewman@users.noreply.github.com>
Co-authored-by: Dimitri Papadopoulos Orfanos <3234522+DimitriPapadopoulos@users.noreply.github.com>