Add x402X extension to README

[Mine77]

Hi Merit magicians,

Thanks for creating the repo.

We are working on the x402X (full name x402-exec), which is an extension protocol that enables an x402 payment to trigger arbitrary on-chain contract calls.
We believe this will open up more possibilities for the protocol such as:

1. split the payment
2. mint token/nft with a payment
3. token swap pre/post the payment

Our current approach is based on a patched x402 package but based on the discussion here, we will be officially relying on the x402 official v2 package.

Relevant links:

• website: https://www.x402x.dev/
• Github Repo: https://github.com/nuwa-protocol/x402-exec