Update RULES.md to describe actual format

[foxcpp]

I am working on implementing the STARTTLS Everywhere list support in chasquid MTA.

I expected RULES.md to be up-to-date, but noticed a couple of issues with it:

• Example file is not valid JSON (there are extra trailing commas all around).
• It says the timestamp format is yyyy-MM-dd'T'HH:mm:ssZZZZ, but the actual value in policies.json includes the sub-second fraction.
• It says it is possible to have Unix timestamps in timestamp fields but that does not seem to be used in practice. This introduces additional complexity in handling, however. It might be worth removing this point.

[sydneyli]

Thanks for the report, @foxcpp! RULES.md has gotten out-of-date, and we need to lint the JSON file for trailing commas.