tell about default in read()
add FEATURES_IMPLEMENTED

We now have a list of features supported by libdiscid on the platform
and a list of features supported by us.
The user can create a list of features currently available with
`set(FEATURES) & set(FEATURES_IMPLEMENTED)`.
We will probably use such a thing internally, but it would probably
be clutter to expose a third feature list in the API.

The list of all features implemented in libdiscid on any platform is not
available at all (if different then ours).