Consider Codesandbox CI for this repo

We've found [Codesandbox CI](https://codesandbox.io/docs/ci) to be pretty handy, allowing contributors to `yarn add <>` a version of a graphql-tools package derived from a PR (effectively a simple forking solution).

Check out the comment on this PR for an example: https://github.com/apollographql/apollo-server/pull/6154