add build instructions

[makayama]

similar to this:

https://bpaste.net/show/82489f780a0a

[mjtorn]

https://gist.github.com/mjtorn/6cf8ed8b33fd91b3ffa05e7981b8f769

I have that, but unfortunately it's on hold until I hear sfdroid builds for the Fairphone2 :/