Add change log

CHANGELOG.md

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+# Changelog
+
+## host table
+
+Any changes to the `host` table with automatically update the `date_updated`
+field. See `update_date_updated` in `PostgreSQL/database-postgresql.sql`.
+
+## 2.4
+
+With this release, you can set a host to be not enabled. Perhaps you have a
+host which is no longer in use but you want to keep the data.
+
+When upgrading to this release, be sure to run these scripts:
+
+* `PostgreSQL/updates-2020.08.28.sql`
+* `PostgreSQL/procedures.sql`
+
+```
+update host set enabled = false where name in ('foo', 'bar');
+```
+
+New versions of the query functions have been created. These take a boolean
+parameter as their last arguement. This allows you to query hosts which are
+no longer enabled (false). The existing functions will now only query
+enabled hosts. You can retain existing behaviour by leaving all hosts
+enabled.