Added flatpak build manifests with split dependencies

[kdcis]

• Added Flatpak build manifests under packages/flatpak/ directory

• Added Flatpak build instructions under docs/ directory

[g1itch]

Also, I don't get a point of documenting the maintenance tasks in such way. Why don't you just make a script buildscripts/flatpak.sh covering the steps under the ##Export header?

There supposed to be a builder on https://git.bitmessage.org/Bitmessage for building and uploading the bundle, which will be used as github check for a new code, like buildbot/PyBitmessage-wine32 and buildbot/dpkg.

[PeterSurda]

@g1itch I'm working on the buildbot integration.