Added citereset to oscola.tex

I have added a quick note about the citereset options to the documentation. It might be worth considering setting "citereset=chapter" by default, especially as the footnote numbering already gets set to 0 at the beginning of a new chapter, and as I cannot imagine a scenario in which it would be desirable to have a cross-reference to a different chapter and different numbers in the output. Unfortunately, I don't have the necessary BibLaTeX skills to do that.