Document CGI libraries that people can use

[technosophos]

Any CGI 1.1 compliant helper library has the potential to work.

I looked at these two C libraries:

• https://github.com/boutell/cgic: This did not compile out of the box mainly because it used mkstemp. However, I could comment out a small chunk of the code and get it working.
• http://libccgi.sourceforge.net/: I could get libccgi to compile out of the box if I turned off OpenSSL integration. However, my C skills are not good enough to do anything useful.

I know there are no libraries for Grain, and I suspect there are none for AssemblyScript. But I will look for Rust