Add note about page design.

docs/PAGE_DESIGN.md

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+Setting up a forum for distributed caching is somewhat complex. It's tempting to
+reach for a Merkle tree or the like, and have each thread have that internal
+structure; this would allow for caching of the entries easily, since each of the
+threads could have effectively a full commit history of the discussion, with
+edits and new comments being handled smoothly. However, this is still a bit
+heavy-weight; since we don't necessarily need the full functionality of a merkle
+tree, and the hashing of history could cause issues with both moderation and how
+quickly a client can load all of the content.
+
+What's probably a good take-away, though, is that we can keep mostly static
+content for each post, and transmit history linearly, but allow for some later
+clean-up. This allows us to simplify some situations, like a highly edited post,
+and allows for removal of information where required (e.g. dmca takedowns).
+