Add some docs on schema migrations.

docs/schema-migrations.md

@@ -0,0 +1,23 @@
+# Schema Migrations
+
+Zulip uses the [standard Django system for doing schema
+migrations](https://docs.djangoproject.com/en/1.8/topics/migrations/).
+There is some example usage in the Zulip new feature tutorial on
+readthedocs.
+
+This page documents some important issues related to writing schema
+migrations.
+
+* **Large tables**: For large tables like Message and UserMessage, you
+  want to take precautions when adding columns to the table,
+  performing data backfills, or building indexes. We have a
+  `zerver/lib/migrate.py` library to help with adding columns and
+  backfilling data. For building indexes on these tables, we should do
+  this using SQL with postgres's CONCURRENTLY keyword.
+
+* **Numbering conflicts across branches**: If you've done your schema
+  change in a branch, and meanwhile another schema change has taken
+  place, Django will now have two migrations with the same number. To
+  fix this, you can just rename the file, as long as no other
+  migrations depend on it (in which case you also need to update the
+  dependencies).