add flask note on naming

doc/apps/flask.md

@@ -18,6 +18,21 @@ To deploy a Flask application in Ploomber Cloud you need:
 - Your application file (`app.py`)
 - A dependencies file (`requirements.txt`)
 
+Ploomber Cloud **expects** your Flask application to be named as the variable `app` within the file `app.py`.
+In order to run and deploy correctly, you must define the app like this basic example:
+
+```py
+# app.py
+
+from flask import Flask
+
+app = Flask(__name__) # Here we define the application as "app"
+
+@app.route("/")
+def hello_world():
+    return "<p>Hello, World!</p>"
+```
+
 ## Testing locally
 
 To test your app, you can use `gunicorn` locally: