add (optional) "entry categories" for supporting different approval processes

[dret]

as discussed with @gklyne (#35 (comment)), it would be useful to mention the ability to have different approval processes for the same registry, for example such as "provisional" and "permanent" registry entries that are used in various IANA registries.
the idea would be that the entries share the same registry and value space, but there are different processes to get into the registry and these are reflected by categorizing the entries accordingly.