chore(docs): add Readme section for GET requests

[jschuur]

This adds a sample implementation of @toinelin's #277, since the ability to make GET requests was otherwise not documented.

Note that you have to specifically pass in a jsonSerializer, or it'll break trying to stringify the query parameters:

const graphQLClient = new GraphQLClient(endpoint, {
  method: 'GET',
  jsonSerializer: {
    parse: JSON.parse,
    stringify: JSON.stringify,
  },
});

This took me a while to figure out, so I thought this would make for a good addition to the docs. Should probably default to defaultJsonSerializer though.

[jasonkuhrt]

ty!