Update README.md with info about running unit tests
Update README.md with info about running unit tests

Created new top level section 'Development', placed existing 'Continuous
Integration' and new 'Running Tests' sections in it. Reordered sections to
place 'Installation' ahead of 'Development'.

mrbean-bremen/pyfakefs,mrbean-bremen/pyfakefs,jmcgeheeiv/pyfakefs,pytest-dev/pyfakefs