clarify that Python must be compiled with shared library support

[copernican]

@jjallaire's comment here states that reticulate needs to bind to the Python shared library, which implies that the shared library needs to exist. If Python is compiled from source without the --enable-shared flag, this will not be the case. I was not able to find this shared library requirement explicitly mentioned in the reticulate documentation, so it might be helpful to add it, possibly in the Python Version Configuration vignette.

[jjallaire]

Thanks for the feedback , done here: 23a61ec