Extend installation instructions to include ARM boards
Extend installation instructions to include ARM boards

Summary:
Extend installation instructions for ARM boards like
Beaglebone Black, Raspberry Pi, ... running a variant
of Debian or Ubuntu. Closes T1220.

Test Plan:
[X] Markdown rendering previewed with Atom.
[X] Decision: Section heading change.
[X] Review: Anything else.
[X] Tested instructions on different arm-based computers.

Reviewers: hendrikk, pmaanen, giso

Reviewed By: pmaanen

Maniphest Tasks: T1220

Differential Revision: https://dev.openmha.org/D435

HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA,HoerTech-gGmbH/openMHA