README.md: some grammar, spelling, and clarity suggestions (#2149)
README.md: some grammar, spelling, and clarity suggestions (#2149)

Note that this adds a link to https://www.npmjs.com/package/apollo-client-preset
but that package has no README. I dropped the missing link on HttpLink (no pun
intended) because I'm not sure what URL its docs will eventually be at.
apollographql/apollo-client,cesarsolorzano/apollo-client,apollostack/apollo-client,apollostack/apollo-client,cesarsolorzano/apollo-client,apollographql/apollo-client,apollostack/apollo-client,cesarsolorzano/apollo-client