Add basic doc for file.contains (#673)

Author: sandzwerg

testinfra/modules/file.py

@@ -116,6 +116,10 @@ def mode(self):
         raise NotImplementedError
 
     def contains(self, pattern):
+        """Checks content of file for pattern
+
+        This uses grep and thus follows the grep regex syntax.
+        """
         return self.run_test("grep -qs -- %s %s", pattern, self.path).rc == 0
 
     @property