Add GRAVEYARD.md

GRAVEYARD.md

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+# bigquery-etl Code Graveyard
+
+This document records interesting code that we've deleted for the sake of discoverability for the future.
+
+## Smoot Usage v1
+
+- [Removal PR](https://github.com/mozilla/bigquery-etl/pull/460)
+
+The `smoot_usage_*_v1*` tables used a python file to generate the desktop,
+nondesktop, and FxA variants, but have been replaced by v2 tables that make
+some different design decisions. One of the main drawbacks of v1 was that
+we had to completely recreate the final `smoot_usage_all_mtr` table for all
+history every day, which had started to take on order 1 hour to run. The
+v2 tables instead define a `day_0` view and a `day_13` view and relies on
+the Growth and Usage Dashboard (GUD) to query them separately and join the
+results together at query time.