Adding initial doc for func subscribe

Signed-off-by: Matthias Wessendorf <[email protected]>

docs/functions/subscribing-functions.md

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+# Subscribe functions to CloudEvents
+
+### Prerequisites
+
+- Knative Eventing installed on the cluster
+
+### Procedure
+
+--8<-- "proc-subscribe-function.md"

docs/snippets/proc-subscribe-function.md

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+<!-- Snippet used in the following topics:
+- /docs/functions/subscribing-functions.md
+-->
+The `subscribe` command will connect the function to a set of events, matching a series of filters for Cloud Event metadata
+and a Knative Broker as the source of events, from where they are consumed.
+
+=== "func"
+
+    To subscribe the function to events for a given broker, run the following command:
+
+    ```bash
+    func subscribe subscribe --filter type=com.example --filter extension=my-extension-value --source my-broker 
+    ```
+
+=== "kn func"
+
+    To subscribe the function to events for the default broker, run the following command:
+
+    ```bash
+    func subscribe subscribe --filter type=com.example --filter extension=my-extension-value 
+    ```
+
+!!! note
+    When invoking `func deploy` the CLI will create Knative Triggers for the function.