Don't use identifiers for non-intended meaning

[garemoko]

The following should not be used as verbs:

• http://id.tincanapi.com/activitytype/tag
• http://id.tincanapi.com/extension/quality-rating

Instead use:

• http://activitystrea.ms/schema/1.0/tag
• http://id.tincanapi.com/verb/rated

[aneesha]

README.md has been updated.

[garemoko]

Looks good. If you're going to use the extension, I'd list this separately from the verbs table to make it clearer.