Add proposal to documentation

[isabela-pf]

Now that we have approval, I'd like to make out original project proposal open for ourselves and others to reference. This PR:

• Adds the proposal text as a markdown document (with personal information removed)
• Creates an images directory to link the one image in the proposal
• Links to this proposal from the readme

Please let me know if you need any other information. Thanks in advance for the review! 🌻

[smythp]

Great to have this in a non-PDF format! Thank you. Looks good to me.

[isabela-pf]

Thank you! I'll merge it at the end of this week if no one else has comments then.

[isabela-pf]

Merging since there's no further comments. Thank you! And feel free to let me know if changes need to be made later on.