doc(api): signals

python/docs/signals.md

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+# Signals
+
+Signals allow certain senders to inform a set of receivers that specific actions have occurred. So a component could be able to send notification to another in a decoupled manner, minimising dependence between two or more components.
+
+We use the Django signals dispatcher module to have signals in Taiga.
+
+You can find more info at [Django Documentation - Signals](https://docs.djangoproject.com/en/dev/topics/signals/)
+
+
+## How to implement Signals?
+
+First, we define some elements:
+
+- **Signal**: An object used to notify a particular event.
+- **Sender**: An object that sends the signal.
+- **Receiver**: A function that will be executed when a signal is dispatched.
+
+So, the steps to use a predefined signal are:
+
+1. Slect a predefinend signal
+2. Define one or more receivers
+3. Connect the receivers with their sender
+
+Predefined signals are related to the ORM Model: pre_save, post_save, pre_delete, post_delete and m2m_changed are some of then. You can find a complete and detailed list in the [Django Documentation about predefined signals](https://docs.djangoproject.com/en/dev/ref/signals/).
+
+If we want to use a custom signal, we follow this steps:
+
+1. Define a signal.
+2. Define one or more receivers.
+3. Connect the receivers with their sender.
+4. Send signals from any component.