add documentation for non-vendored rust bindings

[jmayclin]

Problem:

s2n-tls bindings provide an option to use an existing libs2n artifact.

s2n-tls/.github/workflows/ci_rust.yml

Lines 67 to 73 in 4862e7f

	export S2N_TLS_LIB_DIR=`pwd`/build/lib
	export S2N_TLS_INCLUDE_DIR=`pwd`/api
	export LD_LIBRARY_PATH=$S2N_TLS_LIB_DIR:$LD_LIBRARY_PATH
	cd ${{env.ROOT_PATH}}
	./generate.sh
	ldd target/debug/integration | grep libs2n.so

However this option doesn't really have any documentation.

Solution:

We should add documentation. Ideally in a place in the readme (so it shows up on crates.io) or in a place so that it shows up on docs.rs