Describe how the Haskell packages support multiple versions of the C++ library

[jorisdral]

Right now, one version of a Haskell package supports multiple versions of the C++ library. Alternatively, we could tie releases of the Haskell packages to releases of the C++ library more closely, but I think the current approach is better.

What we should definitely do is describe the relationship between versions of the Haskell packages and the C++ library, and how we in general approach support the C++ libraries.

Some questions we should answer:

• What is the relationship between the version of the Haskell package and the version of the C++ library?
• Do we always provide bindings to functions that are only available on some versions of the C++ library? Or, in other words, how are newly exposed C++ entities supported?
• How do deprecations in the C++ library relate to deprecations in the Haskell package?
• Are the answers to these questions different for different Haskell packages?

[jorisdral]

Note that some of this work is already being done as part of #85