Add information so that people can determine what functions are extended and what additional functions are provided

[MikeFultonDev]

I went to the docs and couldn't figure out if a function was available or not. 3 Examples came up that I tried to find:

• tempnam (which I don't think is supported)
• freopen (which I don't think is supported)
• open and fopen (which were supported but I couldn't find them)

This would seem the most common case someone would go to the docs for and yet I gave up and grep'ed the code.

A simple solution would be to just have a list of functions that extend the core z/OS functions. A link would be great, but even the name of the file would be ok to start.
Also the services you provide would be handy to show too (e.g. __setdcssid).