add validators pkg readme

[ceyonur]

Adds validators pkg readme

[meaghanfitzgerald]

Suggestions are mostly for phrasing and grammar. It would be helpful if the read pointed to the places in Subnet-EVM where these functions exist, and provide example curls (or point to the ones in the docs site).

[ARR4N]

How much existing knowledge is a reader assumed to have? If non-negligible, where can they get that knowledge?

[ceyonur]

How much existing knowledge is a reader assumed to have? If non-negligible, where can they get that knowledge?

I also captured AvalancheGo's uptime manager basics in that readme, so we don't assume any existing knowledge. LMK if anyhing is not clear or you think it requires additional context.

[darioush]

some nits