Update documentation

[supersat]

Currently there is no documentation about how to build the app. There are some hints in the Travis CI build scripts, but it would be nice if we had better documentation.

One thing to note is that newer versions of GCC produce code that is too large to fit on the target osmocom-bb phones. For example, on my Mac, I'm using gcc-arm-none-eabi-4_9-2015q3-20150921 from https://launchpad.net/gcc-arm-embedded/+series