Document SSL headers dependency on Ubuntu

README.md

@@ -47,6 +47,7 @@ First, ensure you have installed the following prerequisites:
 * [Rust](https://www.rust-lang.org/) >= v1.51.0, as installed via [rustup](https://www.rust-lang.org/learn/get-started).
 * [Docker](https://www.docker.com/) >= v20.10.6
 * [Docker Compose](https://docs.docker.com/compose/) >= v1.27.4
+* (Ubuntu only) install `libssl-dev`
 
 Then, run these commands to clone, build, and run the benchmark suite's tests:
 
@@ -78,4 +79,4 @@ $ FHIR_BENCH_ITERATIONS=1000 \
 If you'd like to add support for a new FHIR server, additional operations,
   or otherwise help to build out these benchmarks,
   you'd be very welcome!
-Please see [CONTRIBUTING.md](./CONTRIBUTING.md) for further details.
+Please see [CONTRIBUTING.md](./CONTRIBUTING.md) for further details.