add proposal document template

[gregsdennis]

Resolves #1443

I started working on the propertyDependencies extraction and the template kinda just worked itself out. It turns out leaving questions is easier that actually filling out the sections.

[gregsdennis]

See ☝️ for an example of this template in use.

[gregsdennis]

We're having some debate on the purpose of this document over on #1450. Holding this until that resolves.

[Relequestual]

Based on the discussion and activity at #1450, I feel this PR needs a suplementary document which explains the purpose of the proposal document (or documents if that turns out to be the case).

I would think such discussion needs to be had at #1443 and agreed on before making further changes here.

I'm happy to facilitate these discussions, either in text form or via calls. @gregsdennis @jdesrosiers let me know which you would prefer. (Anyone else is also welcome to be involved, but Greg and Jason are the primary individuals driving this right now.)

[gregsdennis]

@jdesrosiers I've also updated the process document to provide direction around creating an ADR.