Add 'Contributing' section to README.md

[phillxnet]

Thanks to @StephenBrown2 for helping to highlight this issue. Our current README contains only a file-by-file/pkg-by-pkg breakdown of License info, and a quick description. We are missing a 'Contributing' section containing linking back to either our general contributor doc section:

• https://rockstor.com/docs/contribute/contribute.html#developers

or to a dedicated section within our docs developer/contributing docs such as we have for our docs:
https://rockstor.com/docs/contribute/contribute_documentation.html#contributedocs
or rock-ons:
https://rockstor.com/docs/contribute/contribute_rockons.html

See partner issue on the canonical nature of our docs re contributions:
rockstor/rockon-registry#350