[docs] Elm in Gitpod

[JesterOrNot]

Add Elm section to the Docs see #383

[jankeromnes]

This is the first instance where you mention .gitpod.Dockerfile in this guide. Maybe some readers will be new to Gitpod, and have no idea what you're talking about.

Please add a brief explanation of what a .gitpod.Dockerfile is, and how to create/use one (e.g. similar to the Scala tutorial, but no need to write several paragraphs of explanations -- I feel like one or two sentences here should be enough).

[jankeromnes]

Hey @JesterOrNot! 😄 Here is another great suggestion for this guide (maybe for the list of external resources): https://github.com/sporto/awesome-elm#tools

[jankeromnes]

@JesterOrNot What is the state of this PR? Can we merge it soon? 🙂

[jankeromnes]

Dumping some useful feedback about Elm tools here so that it doesn't become lost:

there are a few must-have tools like elm-format and elm-test that almost everyone uses in almost all projects, and a few side tools for things like creating and hot-reloading a project like create-elm-app/elm-live/parcel, and some tools for quality like elm-analyse and elm-review

[ghuntley]

@JesterOrNot What is the state of this PR? Can we merge it soon? 🙂

@jankeromnes I'll take over this pull-request and it's ready for review.