First pass at guidance document for ATDIS vendors
Fix some typos and add an improve this page link
Update _guidance.md

minor wording/grammar tweaks.
Link to the more helpful webpage of the spec doc
Less exclaimy
Add colon
Reorder words to make it read better
Add space so that unordered list renders properly
DRY up text to make it nicer to read
Make full stops and end of list items consistent
Don't have a list when a sentence will do
I think a plural sounds better here
Merge paragraphs
Remove brackets
Give the reader an idea of how many steps up front
I don't think this should be a heading
Add anchor links on every header so you can link directly to them
Add a bit more context
Not normal-human readable but normal, human-readable
Inline quote, link, and provide a bit of help