#178 Update Guidelines to better explain use of charDecl

[Elli Mylonas]

The Guidelines page Symbol (non meaning-bearing) recommends using <charDecl> to define other representations for symbols such as "crux," "hedera" etc. However, it doesn't explain what <charDecl> is or where it is to be found.
Should update the doc with some details, including a link to the TEI Guidelines.