Improve README for Registering/Deregistering Phone Numbers and Looking Up Phone Numbers

[Suvadra-Barua]

Obtaining information from the README is straightforward for developers familiar with minipay/social-connect. However, for newcomers seeking clarity on the process of registering/deregistering phone numbers and looking up phone numbers, understanding the context can be challenging without thoroughly reviewing the entire codebase or consulting a contributing developer.

For example, before attempting to look up a phone number, developers should register as an issuer and DEK, ensure they have cUSD for making requests to ODIS (transforming plaintext identifiers to obfuscated identifiers), and understand limitations such as the maximum number of addresses that can be looked up per cUSD.

These steps are currently undocumented here. If they have been properly documented elsewhere, they should be included in this README.