Future proofing

[dlongley]

The Document Creation Algorithm currently says:

2. Using a colon (:) as the delimiter, split the identifier into its components: a scheme, a method, a version, and a multibaseValue. If there are only three components set the version to the string value 1 and use the last value as the multibaseValue.

We should be careful that our language allows a future version to have more components. If we find that nothing needs to change in the language today to allow for this, we should at least minimally say in a note that a future version could define additional components. It would be better for parsers to error with Unsupported version 'X' rather than Too many components.