Add hwloc build requirement to README.

README.md

@@ -54,7 +54,10 @@ The instructions below assume you have independently installed `rust-fil-proofs`
 
 **NOTE:** `rust-fil-proofs` can only be built for and run on 64-bit platforms; building will panic if the target architecture is not 64-bits.
 
-Before building you will need OpenCL to be installed, on Ubuntu this can be achieved with `apt install ocl-icd-opencl-dev`.  Other system dependencies such as 'gcc/clang', 'wall' and 'cmake' are also required.
+Before building you will need OpenCL to be installed. On Ubuntu, this can be achieved with `apt install ocl-icd-opencl-dev`.  Other system dependencies such as 'gcc/clang', 'wall' and 'cmake' are also required.
+
+You will also need to install the hwloc library. On Ubuntu, this can be achieved with `apt install hwloc libhwloc-dev`. For other platforms, please see the [hwloc-rs Prerequisites section](https://github.com/daschl/hwloc-rs).
+
 
 ```
 > cargo build --release --all