Add BUILD.md with build from source instructions

Add a specific BUILD page into source with build from source instructions. 
I looked at the existing https://fedorahosted.org/scap-security-guide/wiki/downloads information and@redhatrises' wiki version at https://github.com/OpenSCAP/scap-security-guide/wiki/Building-from-Source

Redhatrises showed make commands running from root directory and also building rpms. That looked good so went with that.