Add note about relative paths to readme (#8613)

We haven't been able to repro the issue, but for at least one person
using a relative path here appeared to cause a build failure.

Author: abadams

README.md

@@ -229,7 +229,8 @@ $ cmake --build build
 
 Setting `-DHalide_LLVM_ROOT` is not required if you have a suitable system-wide
 version installed. However, if you have multiple LLVMs installed, it can pick
-between them.
+between them. Do not use a relative path for `Halide_LLVM_ROOT`. It can cause
+problems on some systems.
 
 ### Windows