Mention PHP7 branches in README
Mention PHP7 branches in README

afk11/secp256k1-php,Bit-Wasp/secp256k1-php,Bit-Wasp/secp256k1-php,afk11/secp256k1-php,afk11/secp256k1-php,Bit-Wasp/secp256k1-php,afk11/secp256k1-php,Bit-Wasp/secp256k1-php,afk11/secp256k1-php,Bit-Wasp/secp256k1-php