Docs: required info to build via Bazel in external repo

[jon-chuang]

rustc requires linker

Alternatively, these flags should be passed into the rust_binary rule

[bsilver8192]

I suspect it depends on your toolchain and which cxx features you use. Some toolchains will use a linker that doesn't care about the order by default, and some very minimal uses of cxx don't have the circular linking order either. #1069 adds more explicit documentation about the problem.