Mention not committing built files in README.md

README.md

@@ -22,4 +22,5 @@ New contributions to the library are welcome, just a couple of guidelines:
 - Please ensure you're changing the individual files in `/src`, not the concatenated output in the `Chart.js` file in the root of the repo.
 - Please check that your code will pass `jshint` code standards, `gulp jshint` will run this for you.
 - Please keep pull requests concise, and document new functionality in the relevant `.md` file.
-- Consider whether your changes are useful for all users, or if creating a Chart.js extension would be more appropriate.
+- Consider whether your changes are useful for all users, or if creating a Chart.js extension would be more appropriate.
+- Please avoid committing in the build Chart.js & Chart.min.js file, as it causes conflicts when merging.