Added information about the spring-commands-parallel-tests gem, which adds commands from parallel_tests

Author: ndbroadbent

README.md

@@ -232,6 +232,7 @@ You can add these to your Gemfile for additional commands:
 * [spring-commands-testunit](https://github.com/jonleighton/spring-commands-testunit) - useful for
   running `Test::Unit` tests on Rails 3, since only Rails 4 allows you
   to use `rake test path/to/test` to run a particular test/directory.
+* [spring-commands-parallel-tests](https://github.com/DocSpring/spring-commands-parallel-tests) - Adds the `parallel_*` commands from [`parallel_tests`](https://github.com/grosser/parallel_tests).
 * [spring-commands-teaspoon](https://github.com/alejandrobabio/spring-commands-teaspoon.git)
 * [spring-commands-m](https://github.com/gabrieljoelc/spring-commands-m.git)
 * [spring-commands-rubocop](https://github.com/p0deje/spring-commands-rubocop)