README: add BIP68 (re bitcoinjs#923)

Author: dcousens

README.md

@@ -178,11 +178,12 @@ npm run-script coverage
 ```
 
 ## Complementing Libraries
-- [BIP21](https://github.com/bitcoinjs/bip21) - A BIP21 compatible URL encoding utility library
+- [BIP21](https://github.com/bitcoinjs/bip21) - A BIP21 compatible URL encoding library
 - [BIP38](https://github.com/bitcoinjs/bip38) - Passphrase-protected private keys
 - [BIP39](https://github.com/bitcoinjs/bip39) - Mnemonic generation for deterministic keys
 - [BIP32-Utils](https://github.com/bitcoinjs/bip32-utils) - A set of utilities for working with BIP32
 - [BIP66](https://github.com/bitcoinjs/bip66) - Strict DER signature decoding
+- [BIP68](https://github.com/bitcoinjs/bip68) - Relative lock-time encoding library
 - [BIP69](https://github.com/bitcoinjs/bip69) - Lexicographical Indexing of Transaction Inputs and Outputs
 - [Base58](https://github.com/cryptocoinjs/bs58) - Base58 encoding/decoding
 - [Base58 Check](https://github.com/bitcoinjs/bs58check) - Base58 check encoding/decoding