[GWG] Community Spend Best Practices

Made substantial updates to this documentation:

The ‘best practices’ section is roughly complete. This part may be subjective, so I’d appreciate feedback from different perspectives. I think I’ll want to add a section to make recommendations about what information may be expected in the on-chain description.

The more technical part for submitting a finished draft proposal on-chain is roughly complete. I’ll need to proofread this to refine details.