chore(CONTRIBUTING): explain how to squash and copyedit

- Explain how to squash multiple commits.
- Copyedit.

Closes #740
fix(tagging compiler): Empty transclusions
fix(tagging compiler): Transclusions with an existing ElementBinder
chore(dom): toString for debugging
fix(tagging compiler): Sibling templates
refactor(forms): use errorStates and warningStates

Closes #736
Closes #706
Fixes #724
chore(tests): remove inject() calls