Add Extension Process to the Specification

[dshanske]

Right now, the spec says that individuals who wish to add or discuss extensions should go to the wiki page. Suggesting the process for extensions become part of the spec and that we consider the possibility that some extensions should become part of the spec as a living standard, and some should remain extensions.

[dshanske]

https://indieweb.org/Micropub-extensions#How_to

[dshanske]

Brainstorming about this....proposing the following expansions amendments to the extension process, to be incorporated into the spec.

• Extension proposals MUST have a use case, a basic usage example to show how implementation would be accomplished, and an issue in the GitHub repository for discussion
• Some proposals may be deemed general enough to be incorporated into the specification once broad adoption is reached. Others may be for specific use cases and may be adopted as official extensions, indicating they do not belong as part of the spec.