Add information on how to build libsecp256k1 to the secp README (#1318)
Add information on how to build libsecp256k1 to the secp README (#1318)

* Add information on how to build libsecp256k1 to the secp README

* Rebase and add link to internal docs
bitcoin-s/bitcoin-s,bitcoin-s/bitcoin-s,bitcoin-s/bitcoin-s