.profile/.zprofile instructions

[jarkkojs]

I tried various ways to integrate this and this is what I've ended up putting to my .zprofile:

# rusty-tags
[ `command -v rustc` ] && RUST_SRC_PATH=$(rustc --print sysroot)/lib/rustlib/src/rust/library/
[ -d $RUST_SRC_PATH ] && export RUST_SRC_PATH

I think instructions benefit of some reference model. Here's just one suggestion.