add documentation for running tests with coverage (#8)

README.md

@@ -69,6 +69,12 @@ Run the tests:
 vendor/bin/phpunit tests/Ltb
 ```
 
+If you want coverage analysis, make sure to install `xdebug` PHP extension, and run:
+
+```
+XDEBUG_MODE=coverage vendor/bin/phpunit --coverage-text --configuration phpunit.xml
+```
+
 
 ### Integration tests