Document how plugin vendors package and distribute plugin files

[FeynmanZhou]

According to the notation plugin design in Notation CLI (https://github.com/notaryproject/notation/pull/809), it would be much clear to document how plugin vendors package and distribute in the notation plugin spec and provide recommendations for plugin vendors on how to publish their Notation plugins and share with Notation users.

In detail, the following questions need to be clarified:

• What files are recommended to be packaged into a plugin binary file?
• Which file format will Notation plugin support in the roadmap?
• Execution permission requirements of the plugin file

[FeynmanZhou]

In detail, the following questions need to be clarified:

• What files are recommended to be packaged into a plugin binary file?
• Which file format will Notation plugin support in the roadmap?
• Execution permission of the plugin file