Add documentation on how to use GitPod to develop/run the code

[humphd]

We recently added support for GitPod in #2406. It's not fully ready yet (see #2427), but it's close, and the integration is complete in the repo, see #2427 (comment).

We need to properly document this, so our devs know what this is, what the implications are of using it, and how to make it all work. I especially want to make sure people don't run afoul of any pricing issues (you get 50 hours a month for free, which is probably perfect for students fixing a few issues), and know what they are clicking.

cc @DukeManh, @cindyledev.

[humphd]

• https://www.gitpod.io/docs
• https://www.gitpod.io/docs/getting-started

[humphd]

Might be nice to add a README badge too, see https://community.gitpod.io/t/is-there-any-open-on-gitpod-badge-that-i-can-just-copy-and-paste/989/6

[humphd]

Here are some things I ran into when I tried using it, which we might be able to document:

#2439 (comment)

I also had questions that we should answer:

• how to turn off/stop/delete the workspace?

[DukeManh]

Seeing how many people struggling to get started, I will write a quick doc for this.

[cindyorangis]

Ping me for review