Add note about rspec-2/3 conflict to README

Documents an issue resulting in "Autotest style autotest/rspec_rspec2 doesn't seem to exist. Aborting."

I *think* an alternative to "the only way" would be to change rspec-core-2.99's autotest/rspec2.rb (so named during the rspec-1 to rspec-2 transition) to autotest/rspec.rb. If it works, than the README advice can include something like "if you need to keep rspec-core-2 installed, upgrade to rspec-2.99" or some such.