Explain how to update existing YAML files for extensions

[dthaler]

Fixes #103

[tychonievich]

As I think about this, I've convinced myself that the Guide to Version Numbers, the places extensions are allowed in the Extensions section, and the things extensions are prohibited from doing in the Extension versus Standard section mean that "can be an extension" and "can be a new minor version" are synonyms. That wasn't a conscious decision, but I agree with that result and would be tempted to try to fix and corner-case where they are not synonymous if we find one.

Thus, I think that extension YAML files can be freely updated with new patch and minor versions, but that new major versions require a new YAML file and URI, just like the standard structures can be freely updated with new patch versions, but new minor and major versions require a new YAML file and URI.

[tychonievich]

Superceded by #51