Documentation about Swift scaler

[domminiks]

We must provide clear and concise documentation about the Swift scaler.

It must:

• Have an introduction and general information about the event source and about the trigger (what is Swift, what the trigger does, based on what, etc.)
• Describe trigger parameters (such as containerName, objectCount and swiftURL) and their related details/remarks;
• Describe the authentication methods available;
• Provide examples on how to set up Kubernetes Secrets for OpenStack credentials, how to set auth parameters properly using TriggerAuthentication, and how to set up the trigger (e.g. the ScaledObject) itself;

The documentation should follow the same structure as the other triggers and event sources as here. In the end, this documentation will appear at Keda's official website.

[pdominguite]

The first version of the documentation was created here.