Highlight availability of ParallelTests module (#773)

* Highlight availability of ParallelTests module

Fixes #772

* Add explicit require as a fix instead

Readme.md

@@ -82,6 +82,8 @@ Running things once
 ===================
 
 ```Ruby
+require "parallel_tests"
+
 # preparation:
 # affected by race-condition: first process may boot slower than the second
 # either sleep a bit or use a lock for example File.lock
@@ -97,7 +99,6 @@ at_exit do
     undo_something
   end
 end
-
 ```
 
 Even test group run-times
@@ -258,7 +259,7 @@ TIPS
  - Use [rspec-retry](https://github.com/NoRedInk/rspec-retry) (not rspec-rerun) to rerun failed tests.
  - [JUnit formatter configuration](https://github.com/grosser/parallel_tests/wiki#with-rspec_junit_formatter----by-jgarber)
  - Use [parallel_split_test](https://github.com/grosser/parallel_split_test) to run multiple scenarios in a single spec file, concurrently. (`parallel_tests` [works at the file-level and intends to stay that way](https://github.com/grosser/parallel_tests/issues/747#issuecomment-580216980))
- 
+
 ### Cucumber
 
  - Add a `parallel: foo` profile to your `config/cucumber.yml` and it will be used to run parallel tests