Point to (backend) documentation somewhere

I don't know if I missed something, and how other people notice things, but after reading the readme, I found no information that there are a bunch of backends in the demo directory. Then even later, I found that there is a bunch of information in the wiki page at https://github.com/Immediate-Mode-UI/Nuklear/wiki . It would be nice if I didn't have to go digging for this information with the assumption that I have to go find third party backend implementations, since the Nuklear documentation heavily implies that there are no default backend implementations with it's platform independence, and lack of backend documentation.

TLDR: please
- integrate the github wiki in the repo
or
- at least mention and link to the wiki as additional documentation

- mention that the demo directory has backend implementations

PS: this is the first time I'm using a header-only library so I may be missing some sort of conventions.

Start with the repository README and inspect the demo directory for its backend implementations. Review the linked GitHub wiki to determine how it should be referenced or integrated. Done means the main documentation clearly points users to backend information and identifies the demo backends.

Written by the indexing model from the issue text.