update README reference to project purpose

[apotheon]

Should the README description of the project's purpose be updated? It currently states its existence as a stopgap solution for a pending change in another project's issue, but the issue has been closed as unimportant.

[marshallpierce]

I haven't kept track of the state of serde, so I'm not sure if this is still a good way of solving the problem. I only put this together because users wanted a not-horrible way to integrate base64 into serde... That said, it seems to be used in practice, and it does work, so 🤷

[Sytten]

I think the project should become the "official" way, I spent time searching and it seems like the current best solution

[marshallpierce]

Fair enough. Would you mind preparing a PR updating the README with what you think would have helped you more quickly assess the state of the world with serde and how this library fits in?