Add Holocene design doc (#72)

* Add Holocene design doc

* Some small tweaks/suggestions for Seb's Holocene design doc (#73)

* spellcheck

* define forwards/backwards invalidation

* define principle of fastest derivation

* define "foreign frame"

* Update protocol/strict-derivation.md

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Co-authored-by: Sebastian Stammler <stammler.s@gmail.com>

* Add note about span batch forward-invalidation

* Address initial async feedback & extend

* add comment to L1 origin selection section

* improve section on activation

* improve L1 origin selection section

* add historical notice, some improvements due to design discussion

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Co-authored-by: George Knee <georgeknee@googlemail.com>