Getting started documentation is inaccurate

[obiwanjacobi]

Trying to add a backend and kept running into an error saying that my backend was not in the project.

I was following https://llvm.org/docs/WritingAnLLVMBackend.html#id7 that never mentions that you have to add your target/backend as sub dir in the LLVMBuild.txt in the target directory...
(at least that was the fix for me)

[frustration]When the getting started docs are not accurate it makes it very difficult...[/frustration]