Publicize the libchromiumcontent builds that Atom Shell uses

[anaisbetts]

Related to #53, it'd be awesome if we could add the pre-built libchromiumcontent URL to the README, so that contributing to Brightray would be easier. I can send a PR from the URL dug out of $REDACTED if this is amenable

[joshaber]

Big 👍 to this. I always have to dig the URL out of Electron.

[alspore]

👍 This would be great.