Added note on spec and lib development reciprocity

Also, added my name to list of contributors.
I think "concurrent" development is overstating the specification development process. I suggest something more like the following wording instead:

> The community has agreed on the set of proposals to include in CellML 1.2, as summarised by tracker item 55 (https://tracker.physiomeproject.org/show_bug.cgi?id=55). These proposals collectively represent CellML 1.2 and are what will be implemented in the initial versions of libCellML. This implementation will help test the proposals and in some cases help choose between alternate serialisations of the proposals in CellML XML.