Add documentation for Boba Billing contract

[boyuan-chen]

Purpose

Any contracts that uses our bridge system to send messages to L1 should ask users to cover the cost

How to

BOBA Billing Contract: https://github.com/bobanetwork/boba/blob/develop/packages/boba/contracts/contracts/L2BillingContract.sol Contract Address:

• Ethereum Mainnet: https://github.com/bobanetwork/boba/blob/85b908db21fa183f33d4f99c00d0b653b5919356/packages/boba/register/addresses/addressesMainnet_0x8376ac6C3f73a25Dd994E0b0669ca7ee0C02F089.json#L88
• Ethereum Rinkeby: https://github.com/bobanetwork/boba/blob/85b908db21fa183f33d4f99c00d0b653b5919356/packages/boba/register/addresses/addressesRinkeby_0x93A96D6A5beb1F661cf052722A1424CDDA3e9418.json#L86
• L2s for Alt L1s: https://github.com/bobanetwork/boba/tree/alt-l1-avalanche/packages/boba/register/addresses

Developers should import this contract and collect user fee. The contracts should be verified and we will add your contracts to our whitelist.

Example: https://github.com/bobanetwork/boba/blob/85b908db21fa183f33d4f99c00d0b653b5919356/packages/boba/contracts/contracts/LP/L2LiquidityPool.sol#L630-L631