document generating bindings with prebuilt libs2n

bindings/rust/README.md

@@ -20,6 +20,20 @@ Generating rust bindings can be accomplished by running the `generate.sh` script
 $ ./bindings/rust/generate.sh
 ```
 
+Alternatively, rust bindings can be generated using pre-built libs2n by first compiling libs2n, and then running the `generate.sh` script:
+```
+cmake . -Bbuild -DBUILD_SHARED_LIBS=on -DBUILD_TESTING=off
+cmake --build build -- -j $(nproc)
+
+export S2N_TLS_LIB_DIR=`pwd`/build/lib
+export S2N_TLS_INCLUDE_DIR=`pwd`/api
+export LD_LIBRARY_PATH=$S2N_TLS_LIB_DIR:$LD_LIBRARY_PATH
+
+cd bindings/rust/
+./generate.sh
+```
+This method is useful if you want the bindings to be built with a non-default libcrypto. Currently, the default libcrypto when generating rust bindings is `aws-lc`.
+
 ## Minimum Supported Rust Version (MSRV)
 
 `s2n-tls` will maintain a rolling MSRV (minimum supported rust version) policy of at least 6 months. The current s2n-quic version is not guaranteed to build on Rust versions earlier than the MSRV.