mgr/dashboard_v2: Updated README.rst, added HACKING.rst

mgr/dashboard_v2: Updated README.rst, added HACKING.rst

Moved developer-related documentation from `README.rst` to a
new file `HACKING.rst`.

Converted `frontend/README.md` to ReST and merged it into `HACKING.rst`.

Added updates to the README provided by @rjfd in PR#83
(updated the whole unit tests content to match the latest changes
on unit test execution, updated the development notes about
controller development)

Signed-off-by: Lenz Grimmer <lgrimmer@suse.com>