Add Forking Docs

One part of the CodeCorps missions seems to be breaking down barriers to entry for people trying to get into open source development. We’ve got pretty good for open source first-timers, but nothing on the details of maintaining a fork.

Let’s add a little write up for volunteers who aren’t explicitly part of the project that covers

• Adding code-corps-ember as a new remote
• Knowing when to to do a hard reset or rebase when your fork goes stale
• Maybe a more detailed breakdown on what rebase does?

[hkasera]

@JoshSmith Sure I can try that first 👍

[joshsmith]

And feel free to join us in our Slack if you need more help: http://slack.codecorps.org/