docs: add instructions on running specs

Author: bethesque

DEVELOPER_SETUP.md

@@ -1,6 +1,6 @@
 # Developer setup
 
-* You will need to install Ruby 2.4, and preferably a ruby version manager. I recommend using [chruby][chruby] and [ruby-install][ruby-install].
+* You will need to install Ruby 2.5, and preferably a ruby version manager. I recommend using [chruby][chruby] and [ruby-install][ruby-install].
 * Install bundler (the Ruby gem dependency manager) `gem install bundler`
 * Check out the pact_broker repository and cd into it.
 * Run `bundle install`. If you have not got mysql or postgres installed locally, comment out the `mysql2` and `pg` development dependency lines in `pact_broker.gemspec`, as these are only really required on Travis.
@@ -13,5 +13,19 @@
 
 * The application will be available on `http://localhost:9292`
 
+## Running the tests
+
+To run everything (specs, pact verifications, vulnerability scan...):
+
+`bundle exec rake`
+
+To run a smaller subset of the tests:
+
+`bundle exec rake spec`
+
+To run the "quick tests" (skip the lengthy migration specs)
+
+`bundle exec rake spec:quick`
+
 [chruby]: https://github.com/postmodern/chruby
 [ruby-install]: https://github.com/postmodern/ruby-install