Add the version support policy (#4390)
As discussed at the GraphQL working group (via
https://github.com/graphql/graphql-wg/issues/1614 and at various
meetings) and as agreed in last weeks meeting, the GraphQL.JS version
policy as drafted by @JoviDeCroock has been approved and aligns with the
informal policy we already had in place. As such, I have added it to the
README.
This PR also promotes a few headings (H3 -> H2) to make more structural
sense in the TOC.
