Update Readme: mention to replace HttpLink with createUploadLink

[bruvers]

The current Readme mentions that any HttpLink links should be removed from ApolloClient. This makes it sound like blocking requirement for some even though createUploadLink can be used to do a similar job that HttpLink did.

I suggest to update the Readme and mention that createUploadLink has similar features as HttpLink and can in many cases replace it.

[jaydenseric]

Agreed.

The repo about text and package description says:

A terminating Apollo Link for Apollo Client that allows FileList, File, Blob or ReactNativeFile instances within query or mutation variables and sends GraphQL multipart requests.

That sounds like it only sends multipart requests. In reality it automatically sends either normal GET/POST requests, or multipart requests when files are used in variables.

I'm really busy on other OSS work, but next time I do some maintenance here I'll improve the wording :)

[jaydenseric]

Thanks for the input; this should be much clearer now in v15 🚀