Clarify version compat between libcxxwrap-julia and CxxWrap.jl

[sloede]

Fixes #143.

Let me know if you would like me to change anything. I just thought this might be useful to document in a central place, considering that the current mismatch between the latest releases of both repositories might trip up other users as well.

[sloede]

On a different note: Great package, it's amazing what we can do with it for a rather involved C++ library 👍 👍

[barche]

Perfect, thanks!