Improve documentation for how to use gallery

[keyboardkat]

Hi guys! Loving Makerstrap, so here are a few initial edits I was wishing for while making Event Reporter starter resource with it: https://thimble.webmaker.org/project/43402/remix

First: It might be because I'm being dense, but while I found all the other parts of the Makerstrap demo page http://mozilla.github.io/makerstrap/demo/#/ really user friendly, the gallery bit eluded me -- and it remained difficult to parse even in the repo itself.

Not sure how hard this would be to do, but creating a Thimble copy-and-paste option to embed galleries would be an amazing addition to the demo page (or, alternatively, more comprehensive documentation on how to set one up ourselves thru the repo).

[k88hudson]

The gallery is kind of a work in progress, it definitely needs some documentation