Explain RSpec config options [skip ci]

Will still work with RSpec runner/config code runs

README.md

@@ -57,6 +57,17 @@ To mutation test Rails models with rspec comment out ```require 'rspec/autorun'`
 RAILS_ENV=test bundle exec mutant -r ./config/environment --use rspec User
 ```
 
+Passing in RSpec Options
+------------------------
+
+**NOTE: Experimental**
+
+You can control some aspects of RSpec using the `SPEC_OPTS` environment variable as usual. If you want mutant to only pay attention to specs in a certain directory, you can run
+
+```sh
+SPEC_OPTS="--pattern spec/subdir_only/**/*_spec.rb" mutant --use rspec SomeClass
+```
+
 Limitations
 -----------