docs: add documentation about DB migration

doc/explanations/db-migrations.md

@@ -0,0 +1,44 @@
+# Database migrations
+
+We use [peewee_migrate](https://github.com/klen/peewee_migrate) to handle our SQL database migrations.
+
+## Create a migration
+
+You should create a migration if you update the SQL database schema.
+To create a new migration, use:
+
+`make robotoff-cli args='create-migration MIGRATION_NAME --auto'`
+
+You should use an identifiable name (such as "add_bounding_box") to the migration.
+
+The `--auto` flag is used to ask `peewee_migrate` to scan all table definitions in the source code and create the migration file
+automatically. You can skip it if you want `peewee_migrate` to create a blank migration file.
+
+
+## Apply the migration
+
+To apply all pending migrations, use:
+
+`make migrate-db`
+
+
+## What if I had a previous installation of Robotoff with an initialized DB?
+
+You should then create yourself the migration table:
+
+```sql
+CREATE TABLE IF NOT EXISTS "migratehistory"
+(
+    "id" SERIAL NOT NULL PRIMARY KEY,
+    "name" VARCHAR(255) NOT NULL,
+    "migrated_at" TIMESTAMP NOT NULL
+)
+```
+
+Then add manually the the initial migration file in database:
+
+```sql
+INSERT INTO "migratehistory" VALUES (1, '001_initial', CURRENT_TIMESTAMP);
+```
+
+Then you can apply remaining migrations with `make migrate-db`.