Add documentation on how caching is handled.

docs/how-is-caching-handled.md

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+# How is caching handled?
+
+In the readme it says "Aggressively cache images.". What does this mean?
+
+This library treats image urls as immutable.
+That means it assumes the data located at a given url will not change.
+This is ideal for performance.
+
+The way this would work in practice for something like a user profile picture is:
+
+- Request user from API.
+- Receive JSON representing the user containing a `profilePicture` property that is the url of the profile picture.
+- Display the profile picture.
+
+So what happens if the user wants to change their profile picture?
+
+- User uploads a new profile picture, it gets a new url on the backend.
+- Update a field in a database.
+
+Next time the app is opened:
+
+- Display the cached profile picture immediately.
+- Request the user json again (this time it will have the new profile picture url).
+- Display the new profile picture.