Add payment examples and clarifications about limits

[yahgwai]

Eg. from a recent conversation:

there's no maximum imposed by any.sender or the metatx libs. The only limit is the block gas limit . You can estimate the amount of gas that would be used by calling provider.estimateGas (web3 has a similar function I think) - so you can check that it's below the block limit of 10,000,000 gas.

Regarding the minimum amount of ether, there's no explicit limit. But you should send enough to cover your transactions, and the more you send in one go the less often you have to top up. Another thing to note is that any.sender requires a bit of a buffer to process transactions - this is due to the fluctuation in the gas fee market. So we lock a portion of your balance when you send us a transaction, then we unlock it again after the transaction is mined. I believe we currently lock at (current gas price + 100 Gwei), so to estimate roughly how much you'll need to have for a tx to execute you should calculate:

1. The amount of gas for the tx = tx_gas
2. An estimate of the current gas price = gp
3. Calculate the lock price = tx_gas * ( gp + 100Gwei)

So for a tx with gas 400,000 and a current gas price of 40 Gwei, you should send at least: 400,000 * (140 Gwei) = ~0.06 ETH.

But i would send more, since the gas market will change and you dont want to have to top up every time. You can practice on our ropsten network to get the hang of the idea.