Update CCN RFC, and introduce a decision log

[twof]

This proposal started out with a very simple premise and implementation, and has gotten more complex as
the community has explored edge cases and facets about how GraphQL is actually used in practice. For
example this proposal starts out by talking about accommodating the "best practices" that are recommended
by the GraphQL documentation and the community, but we discovered pretty early on that there are
legitimate use cases where the "best practices" are rightfully ignored.

In order to cover instances like that, we've needed to justify additional complexity which can be
difficult to understand for newcomers without (at this point a full year of) context. This decision
log was written with newcomers in mind to avoid rediscussing issues that have already been hashed out,
and to make it easier to understand why certain decisions have been made. At the time of writing,
the decisions here aren't set in stone, so any future discussions can use this log as a starting point.