Indicate that rb-inotify and rb-fsevent aren't actually necessary

Gemfile

@@ -6,5 +6,8 @@ gem 'neat', '~> 1.3.0'
 gem 'colorize', '~> 0.5.8'
 gem 'launchy', '~> 2.1.2'
 gem 'sys-proctable', '~> 0.9.3'
+# These gems aren't actually required; they are used by Linux and Mac to
+# detect when files change. If these gems are not installed, the system
+# will fall back to polling files.
 gem 'rb-inotify', '~> 0.9'
 gem 'rb-fsevent', '~> 0.9.3'