Add documentation on how to propose/add new CUBA

[nathanfranklin]

Add documentation on how to propose/add new CUBA. For example:

• how does one change cuba.yml
• what scripts should be run (as cuba.yml is used to generate code and documentation)
• how one goes about creating a PR and getting the proposed CUBA words approved

[nathanfranklin]

@ahashibon and I were thinking either a script that would contain the needed steps or the needed steps would be clearly listed so that someone could easily copy/paste them to their terminal.

[nathanfranklin]

Closed by #247