Add references about how to get into MetaMask's token lists

[vandan]

Add guidelines and references for getting on MetaMask's token lists into the Display Tokens HowTo article.

References:
https://github.com/MetaMask/contract-metadata/#submission-process

[vandan]

As part of this update, we may also want to consider adding a simple playbook for L2s to onboard with MetaMask.