docs(history): begin collecting things of historical note

I think it's nice to have a place to keep around things that are
relevant, but not necessarily accurate anymore. We could also expand
this to actually describe the history of depot, but I lack the context
for that as I haven't been around from the beginning.

We may want to redirect the link from README to the monorepo doc here.

Change-Id: I971f4ab873a66ba66719a0795d86b4de0e32d32b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13039
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

docs/history.md

@@ -0,0 +1,18 @@
+Historical Notes on depot
+=========================
+
+This document collects references to documentation, design documents,
+code etc. that are no longer accurate nor used, but are of interest as
+historical background for the current state of depot. A comprehensive
+overview of the history of depot remains future work.
+
+- [The TVL Monorepo](https://tvl.fyi/monorepo-doc),
+  original (?) living design document for depot.
+- [Sparse Checkouts](designs/SPARCE_CHECKOUTS.md),
+  a proposal for a tool to create sparse checkouts of depot.
+
+<!--
+TODO(sterni): include other Google Docs I have links to?
+Questionable whether it is worth it in all cases, as some of them may
+have never been shared beyond ##?tvl(-dev)?. Interested in opinions.
+-->