Corrected Markdown format and added links to JHBuild in setup guide for GJS.

Oh gosh hi! I hope this request is okay, if not I'd definitely appreciate feedback on what's acceptable and what's not. The first thing I noticed while working on setting up GJS was the Markdown formatting on the links, or at least what I thought some were links.

This is what I saw:

For the time being, we recommend using JHBuild to develop GJS. Follow the [instructions from GNOME][jhbuild].

This is what I changed it to:

For the time being, we recommend using JHBuild to develop GJS. Follow the [instructions from GNOME](https://wiki.gnome.org/HowDoI/Jhbuild) for [JHBuild](https://git.gnome.org/browse/jhbuild/).

The end also seemed to indicate linking:

[jhbuild] https://wiki.gnome.org/HowDoI/Jhbuild

So I fixed it to:

[JHBuild](https://wiki.gnome.org/HowDoI/Jhbuild)

Hopefully that could be clarified if I was correct in my assumptions or completely wrong in my changes. I figured it would possibly be helpful with correcting the link format in Markdown and adding links to the wiki and the project, too. Hope I didn't mess this up completely, thank you all so much!