This adds a very basic OsmoHNBGW user manual with an "overview" and "running" chapter. It still misses any information on configuration. Change-Id: I54825a4ce5a457f87b0618332347eed8ea54afcb Closes: OS#2588
Fixes: OS#2589 Change-Id: I042734fa9979b598d5471839e408cbf39bba57b5