README: update recommended excludes

By default the `.git` directory will also be scanned forPHP files and as it often contains a lot of directories, this can really slow down the run time for the linting.

By adding an exclude for the `.git` directory in the Symfony example, this will hopefully help people realize that excluding the `.git` directory is a good idea.

Author: jrfnl

Diff for: README.md

@@ -91,7 +91,7 @@ It is strongly recommended for existing users of the (unmaintained)
 
 To run from the command line:
 
-    vendor/bin/parallel-lint --exclude app --exclude vendor .
+    vendor/bin/parallel-lint --exclude .git --exclude app --exclude vendor .
 
 
 ## How to upgrade