Add note about needing pkg-config

On Linux, you can typically install OpenSSL via your package manager. The

headers are sometimes provided in a separate package than the runtime libraries

- look for something like `openssl-devel` or `libssl-dev`.

- look for something like `openssl-devel` or `libssl-dev`. You will also need the

regular development utilities, like `pkg-config`, as the custom build script relies 

on them.

```bash

# On Ubuntu