Maintenance 0.3.8

Pure maintenance release, got a complete number instead of a suffix as npm did not like 0.3.7-a which contained a real bugfix and should have been 0.3.8 in any case.

• Documentation of knx_config.json changed: Recommend to place all services in a room into one device. Issue #83
  Thanks to Matevž Gačnik for pointing that out.
• Version bumped to 0.3.8 as npm does not like 0.3.7-a (no update on `npm update homebridge-knx, see #82)