make pairing header linkable
make pairing header linkable

This makes the "pairing" section linkable via simply `#pairing` – I'd like for each of the libraries to follow this pattern.

Similarly, I'd like each of the libraries to expose a standard "Prerequisites" section following this files' format, with the links included.
kleetus/java-bitpay-client,bitpay/java-bitpay-client,ajp8164/java-bitpay-client