feat(echo): document correct usage of ethers v6

[merklefruit]

From one of our users:

Would be good to add the ethers 6 issue to echo, since as more people start to use ethers 6, more users would be directly asking your support for the same issue with the same questions.

The issue is the following:

When upgrading from Ethers v5 to v6 (in typescript) it's important to remember to fill in the chainId and nonce fields of the transaction when building it. This was not necessary in Ethers v5 but is now. Failure to do so will mean that no transactions will be mined.