Improve Documentation on how to add a new Language

[thekief]

Is your feature request related to a problem? Please describe.
n/a

Describe the solution you'd like
I am fully aware that you will not add any further languages, so I looked into adding it locally but I could not get it running. To be more specific, I tried to add LLVM IR highlighting and while highlight.js seems to support it, codemirror does not.

As I am not too familiar with the carbon project itself nor its dependencies, it would be nice to have a small guide or hints on how to add languages such as LLVM IR. I really tried to understand it but I always get stuck with either codemirror not finding the language, or the highlighting not working.

Describe alternatives you've considered
Many, but none of them being more useful in providing the highlighting.

Additional context
n/a