Add only-percentage docs and example

Author: ttomdewit

README.md

@@ -22,8 +22,11 @@ Run the script:
 ```bash
 vendor/bin/phpunit --coverage-clover clover.xml
 vendor/bin/coverage-check clover.xml 80
+vendor/bin/coverage-check clover.xml 80 only-percentage
 ```
 
+With the `only-percentage` enabled, the CLI command will only return the resulting coverage percentage.
+
 It's also possible to add the coverage report generation to the phpunit.xml.dist add to following line to the xml file:
 ```xml
 <logging>