Improve readme

[sbliven]

Improve the README with clear installation and usage instructions for CE-Symm.

A user reported being unable to run CE-Symm:

Unfortunately the software still does not run for me and hangs when analyze is clicked. Could the readme.md be updated to include any dependencies?

I downloaded the software from
https://github.com/rcsb/symmetry/releases/tag/cesymm-2.0.0

I tried installing on both mac and linux. In both cases the initial box pops up, and I can select custom file, but when I click analyze the software does nothing.

For mac I installed the following additional software steps.
I updated my java runtime environment to Java8 update 191
I installed biojava-core-5.1.1.java in Library/Java/Extensions/biojava-core-5.1.1.jar

On mac the software hangs and it seems like the analyze button does nothing.

On Linux I get the following error:
Exception in thread "Thread-25" java.lang.IllegalArgumentException: Empty Atom array given.

This is probably primarily a documentation problem (for instance, I'm not sure where the Extensions directory came from).