Improved flow and consistency of Contribution guide

Made some improvements to the Contribution guide, `CONTRIBUTING.md`:

* Reformatted and reordered tag tables for readability and to put required tags at top.
* Added minor explanation on the use of processing vs. attribute tags in `conform`.
* Normalized use of “field” and “tag” terms to reduce confusion: “field” now always refers to address data and “tag” now always refers to parts of the source JSON file.

/cc @ingalls @iandees