Change README to instruct users to install OpenSSL, not LibreSSL

The CI and builds were changed, and the code now depends on OpenSSL, and will not build with LibreSSL. Thus, we should update the README to have correct build instructions again.

Open the README and locate the build instructions mentioned in the issue. Update them to tell users to install OpenSSL rather than LibreSSL, then verify that the instructions consistently reflect the dependency required by the current builds.

Written by the indexing model from the issue text.