Move docs content to README.md

[cannin]

CHECK issue (see https://github.com/cytoscape/cyjShiny/actions/runs/1159662734)

Non-standard file/directory found at top level:

'docs'

I suggest moving the contents to README.md. Documentation content in other places wiki and docs is minor and often gets forgotten. I recently was contacted by someone who was still building the package with old wiki instructions.