Document CLI UI guidelines

[microwavables]

@aaronshurley commented on Tue Mar 02 2021

Flag guidelines:

• --dangerous-*
• --*-alpha

Examples of current inconsistencies:

• running kapp produces help msg and error
  • vendir and imgpkg on the other hand just produce error message with list of sub commands
  • ytt and kbld return successfully (since they have root commands registered)

Note: This issue originated from https://github.com/k14s/design-docs/issues/2 but has been moved here as a part of the carvel rename.

[microwavables]

Moved from /carvel-community to /carvel as we are merging the two repos to have /carvel be the main one