Add documentation for inline use of <supplied>, <surplus> and <sic>

[stenskjaer]

I want to have the results from the discussion on unresolvable problems (#5) added to the documentation, and I sort of accept that maybe <sic> is the best we have (although I don't support the idea of using @rend="cruc" on the element, as noted in the issue). And then it struck me that maybe this could be included in a section of situations where marks in the text (such as <added>, [deleted] and then †crux†) are sufficiently clear for indicating the intention of the editor.

I have suggested this section as I consider the problem of the crux to be a standard and common problem in textual criticism that I believe that LBP-schema should be able to deal with.

You are welcome to reject the suggestion. Alternative I wonder whether there should be added a description of <sic> in the context of the critical apparatus too (since the other two cases are actually already covered in that context, and you might want to make notes on these things).

This includes description of the general phenomena of simple critical changes that don't require a critical apparatus enty as well as description, rules and examples of the thee suggested elements.

If accepted, this closes #5.

[jeffreycwitt]

Awesome, looks good. Accepting now.