docs for updating grammar

Author: alunny

README.md

@@ -22,6 +22,21 @@ Allows you to edit xcodeproject files and write them back out.
         console.log('new project written');
     });
 
+## Working on the parser
+
+If there's a problem parsing, you will want to edit the grammar under
+`lib/parser/pbxproj.pegjs`. You can test it online with the PEGjs online thingy
+at http://pegjs.majda.cz/online - I have had some mixed results though.
+
+Tests under the `test/parser` directory will compile the parser from the
+grammar. Other tests will use the prebuilt parser (`lib/parser/pbxproj.js`).
+
+To rebuild the parser js file after editing the grammar, run:
+
+    ./node_modules/.bin/pegjs lib/parser/pbxproj.pegjs
+
+(easier if `./node_modules/.bin` is in your path)
+
 ## License
 
 MIT