Section on using Flatpak as a developer platform

[allanday]

So far, the docs are mostly about building and distributing apps. It would be great if we could promote Flatpak as a developer technology also. Things that could be included here:

• Using Flatpak to create nightlies or testing versions
• Using an SDK as the primary development environment
• Using GNOME Builder

[csoriano1618]

Another point would be how to use Flatpak + CI for a devops-ish workflow.

I wrote this for GNOME, which has both the common flatpak bits and the specific GNOME Sdk bits. Eventually the common part can live in Flatpak docs themselves.

As a point of reference, snapcraft documents their CI integration.