Add suggestion to Factory Reset Chromecast after registering to get it to show up

[joshstrange]

I've read a number of issues here about not being able to add a Chromecast to Greenscreen but most of the issues were fixed by just waiting. I waited until the last minute to play with this before demoing it for potential use at my company (started last night, demoing today, yes I know, I should have started earlier) and after waiting about 9-10 hours I still couldn't add my Chromecast. I stumbled across a SO post suggesting doing a factory reset (as the 'Send this Chromecast's serial number to Google when checking for updates' checkbox no longer exists AFAICT) and I tried that and I was instantly able to add my Chromecast.

Just thought it might be helpful to add that into the README. (I'm willing to do it as long as the pull request process isn't too arduous, I don't see a CONTRIBUTING document and there are only 3 committers to this repo and I don't really want to put my name up with theirs for just adding a tiny bit of text)

[nelsonjchen]

This worked for me! Thanks. You should just put out a PR!

[ehzhang]

@joshstrange a PR would be great :)