Want to add my servant example for versioned APIs and partial client implementations

@fisx @soenkehahn

A few people I've spoken to didn't realize you could do a versioned API or have a partial client implementation in Servant. I didn't either, but I took a look at it with @fisx and it's quite easy. I thought it would be worth having an example project in haskell-servant and linking to it in the Servant documents. I already have a working example project with tests. If you are willing to add it I'll clean the package up and add a more documentation.

https://github.com/mchaver/example-servant-partial-client