release of 2.2.1, edited README

Minor dialog edit.

CellProfiler Analyst 2.0.4, SVN rev. 11306

CellProfiler Analyst 2.0.3, SVN rev. 10530

CellProfiler Analyst 2.0.2, SVN rev. 9128

CellProfiler Analyst 2.0.1, SVN rev. 8634

CellProfiler Analyst 2.0.0, SVN rev. 8582