Add Crypto policy

[Exairnous]

@hobbs-Hobbler and @nurse-the-code have expressed that the word "decentralized" in our vision and guiding principles document is sometimes used with crypto projects and we don't want to turn-off crypto-skeptic folks and desire a policy to define the Hubs Foundation's stance on the integration (or not) of crypto functionality into Hubs.

@Exairnous notes that we should be careful not to violate our "unopinionated" principle.