Make LGPLv3 or GPLv2 constraint clear in the README

[mratsim]

Due to the GMP dependency which is LGPL v3 or GPL v2, a binary build of libsnark has firm constraints and cannot be distributed with binary Apache v2-licensed code or is incompatible with app stores.

Note that only binary builds which becomes derivative works of LGPL/GPL have this constraint, sources do not have redistribution issues.