Document loading keys from env file

This was added in laravel/passport#683

Author: driesvints

passport.md

@@ -184,6 +184,11 @@ If necessary, you may define the path where Passport's keys should be loaded fro
         Passport::loadKeysFrom('/secret-keys/oauth');
     }
 
+Additionaly, you may load the keys from the `.env` file directly:
+
+    PASSPORT_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----\n<private key here>\n-----END RSA PRIVATE KEY-----"
+    PASSPORT_PUBLIC_KEY="-----BEGIN PUBLIC KEY-----\n<public key here>\n-----END PUBLIC KEY-----\n"
+
 <a name="configuration"></a>
 ## Configuration