add ADR for including order in digests

[nsheff]

Explains the decision that the final digests MUST reflect sequence order (but does not specify exactly how).

[nsheff]

@tcezard and @andrewyatz Can you approve this?

[andrewyatz]

All looks good and appears to be a truthful summary of the discussions.

[nsheff]

@sveinugu does this accurately reflect what you think about this topic as well?

[sveinugu]

@sveinugu does this accurately reflect what you think about this topic as well?

Not sure whether we have concluded that the top-level digest is required to be ordered, which seems to be what the decision entails.

Also "collections that identical" -> "collections that are identical"

[nsheff]

Not sure whether we have concluded that the top-level digest is required to be ordered, which seems to be what the decision entails.

@sveinugu I think it's not saying anything about the level, just that the final solution must be able to account for different orders, so I think it accurately reflects what we've agreed.