Add README to database upgrades directory

etc/db/upgrades/README.md

@@ -0,0 +1,34 @@
+# Database schema upgrades
+
+When the database schema needs to be upgraded in order to provide new functionality, fix errors, etc., an upgrade script should be added to this directory.
+
+The script can be SQL (preferred) or anything else (Bash, Python, …) in the event of complex upgrades.
+
+The script itself should:
+
+* document what the intended changes are;
+* contain instructions on how to run it;
+* make the user aware of any non-obvious side effects; and
+* say if it is safe to run the script multiple times on the
+* same schema / database.
+
+## Naming
+
+Script files should be named `upgrade-<index>-<commit-id-old>-<commit-id-new>-v<schema-version>.sql`, where:
+
+* `<index>` is a correlative two-digit index. When reaching 99, existing files will be renamed to a three digit index (001-099) and new files will use three digits.
+* `<commit-id-old>` is the ID of the Git commit that last introduced a schema change.
+* `<commit-id-new>` is the ID of the first Git commit expecting the updated schema.
+* `<schema-version>` is the version of the schema.
+
+Note: the `<schema-version>` value should be updated with every change and it should be the same as reported by:
+
+```sql
+select value->>'db_schema' as db_schema from public.info where key = 'version';
+```
+
+If necessary, the wanted schema version must also be updated in `package.json`.
+
+## Running
+
+Schema upgrades are always run manually.