Documented mandatory core fields - compression is actually optional.
Explained and reordered conform info to put common attributes at the top
Reformatted core fields table
Reformatted conform processing fields table
Reformatted conform attribute fields table
Normalized usage of "tag" and "field" terms

"Field" now always refers to a value from the source data.
"Tag" now always refers to a value in the JSON source file.
Reformatted parts of markdown and text for readability
Normalized usage of "column" and "field" terms

All columns are now consistently known as fields.
Merge pull request #738 from openaddresses/update-conform-info

Improved flow and consistency of Contribution guide