Added libcap to the software requirements section of README.md.

It appears to be required for 'make install'; I believe it's used to let unprivileged users run lispd.
I put down 'libcap v2+' in the first bullet point list since I think that some distributions, [like Arch Linux](https://www.archlinux.org/packages/core/i686/libcap/), don't call the package 'libcap2', but just libcap and set the version field to something bigger than 2 (or at least that's what it looked like). I didn't go and check if libcap v2.0 is actually good enough, but I'm not sure if that level of granularity is required here.