Add instructions for compilation under Ubuntu 18.04

[pnprog]

Just tested this on a fresh Ubuntu 18.04 LTS. Current PPA cannot be used with Ubuntu 17.10 and following.

[lemonsqueeze]

Thanks, will be helpful for newcomers. I tweaked the dependencies slightly:

• libcaffe-cpu-dev instead of libcaffe-cuda-dev so it works everywhere
• libopenblas, better perf than libblas

[lemonsqueeze]

Merged as 9cb2e95.