add documentation notes on sql migration gear

dev_utils/docs/sqlalchemy_database_migration_gear.md

@@ -0,0 +1,48 @@
+# SQLalchemy database migration gear
+
+### apply migrations
+
+export the app script path as bash variable
+
+    export FLASK_APP=chuck_norris_jokes_caller/app.py
+
+Initialize the db migration gear: it will create a folder at path `migrations`.<br>
+
+> [!TIP]
+> You should ignore the error in case a folder `migrations` is already present/versioned in the project.
+
+> [!NOTE]
+> In case you deleted migrations and you want to init the db again:
+> 
+>     sqlite3 chuck_norris_jokes_caller/chuck_norris_jokes_caller.sqlite
+>     
+>         delete from alembic_version;  
+
+create migration required files
+
+    flask db init
+
+save the changes to DB structure
+
+    flask db migrate -m "< migration message here>"
+
+apply the changes to DB structure
+
+    flask db upgrade 
+
+> [!NOTE]
+> Every time the DB structure is changed, you have to run
+> 
+>     flask db migrate -m "< migration message here>"
+> 
+>     flask db upgrade
+
+> [!NOTE]
+> Every time you open a bash session or terminal and want to operate the migrations, you have to run
+> 
+>     export FLASK_APP=chuck_norris_jokes_caller/app.py
+
+
+> [!WARNING]
+
+> migrations do not affect changes in properties of field such as primary key, uniqueness, blankness, non-null ness...