Address addition of new features

[ddorwin]

The Other and related mechanisms section says, "Feature Policy... provides an extensible mechanism," and the directive definition says, "Future versions of this document may define additional such members and user agents must ignore unknown members when parsing the object." However, there does not appear to be prose about adding new features or how implementations should handle unrecognized features, including those added by new versions of the spec.

It's probably worth addressing backwards compatibility considerations, adding Feature Policy checks to specs early, instructions for adding new features to this document, etc.

Such text may also want to recommend default policies for new features, which I believe would be [self, null] and null.