Document SemVer API surface

[ChristianMurphy]

Document how changes affect the versioning.
Provide some examples.

[ChristianMurphy]

Initial thoughts.
The API surface for web components is defined by:

1. the CSS variables
2. the HTML attributes

Changes that don't affect either are patches.
Changes that add new variables/attributes are a minor release.
Changes the remove variables or attributes are a major release.

3. The framework version.

New major versions of Vue may need to be part of a major release here.
The main reason to consider it a part of the API, is currently that Vue version needs to be defined outside the component.
If components were bundled with Vue, the Vue version would have not have an effect.