Add build instructions to README.md

[anadon]

A few smaller things with documentation. First, yes we all know how to use and run cmake, but it should be there anyways because people do forget and less experienced people do come along and it makes it much easier on them. Second, README.md should be more readable from the command line.

[tonyelewis]

Thanks for contributing to cath-tools.

The documentation for building cath-tools is in the Docs under "Build".

TBH, I haven't made that documentation very prominent because I expect the strong majority of users to just use the Executable Downloads rather than building the code themselves.

I agree that the README.md isn't very readable on the command line, but that's a trade-off against how helpful it is on the GitHub front page and Read the Docs front page and I expect the strong majority of users to viewing it there.