Updated boolean validation doc

Closes #414

Co-Authored-By: John Austin <[email protected]>

Author: GrahamCampbell

README.md

@@ -227,7 +227,7 @@ One or more environment variables failed assertions: FOO is not an integer
 
 ### Boolean Variables
 
-You may need to ensure a variable is in the form of a boolean, accepting "On", "1", "Yes", "Off", "0" and "No". You may do the following:
+You may need to ensure a variable is in the form of a boolean, accepting "true", "false", "On", "1", "Yes", "Off", "0" and "No". You may do the following:
 
 ```php
 $dotenv->required('FOO')->isBoolean();