updated README with instructions about secp256k1 dependencyfor MacOSX
Merge pull request #43 from gubatron/master

updated README with instructions about secp256k1 dependency for MacOSX
libsecp256k1 now has a pkg-config file so rely on that for autodetecting the pkg.
add required libsecp256k1 to make flags
Patch secp256k1_start() for breaking change.