Update README.md for local dev instructions

[lukechurch]

@devoncarew PTAL - I'm not sure the way I'm suggesting people use grind is correct, I've had some problems running the travis script locally. Could you correct as appropriate?

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project, in which case you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[lukechurch]

@googlebot I signed it!

[devoncarew]

This isn't necessary to run the front-end; you can launch web/index.html (the desktop version) or web/mobile.html (the mobile version) directly from the editor.

I think these instructions may confuse people a bit. Perhaps add a disclaimer that the front-end can be run w/o any server, but if you want to test the auto-redirect, here are some steps?

[lukechurch]

Yup, sg, will do.

[devoncarew]

lgtm w/ comments

Please make yourself right with @googlebot

[lukechurch]

Thanks @devoncarew .

Issues with @googlebot seem to be related to using GitHub's UI for modification, I'll route things through a local change in future.

[devoncarew]

still relevant?

[lukechurch]

@devoncarew Lets chat about this in a couple of weeks and review. I still have a bunch of problems with my local build setup seemingly.