Add documentation for Identifier Schemes

[cookeac]

The various derivatives of icarIdentifierType are not well documented, and while there is documentation about schemes in the Wiki (https://github.com/adewg/ICAR/wiki/Schemes) it does not lend itself to maintenance:

• Only those with write access to the repo can edit
• It is not visible in forks
• It is not versioned with releases.

This should be replaced with a link to markdown documentation and identifier lists within the code.