Document safetrace/enclave dependencies for libzmq and llvm-config

[sbellem]

As I went through the installation and build steps for the safetrace/enclave, I was missing libzmq3-dev (ubuntu 18.04) and had to symlink llvm-config-6.0 (ln -s /usr/bin/llvm-config-6.0 /usr/local/bin/llvm-config).

I could make a pull request documenting these dependencies and details if it makes sense. Perhaps, libzmq3-dev can be added to the script shown under https://github.com/enigmampc/SafeTrace/blob/master/enclave/INSTALL.md#software ...?