[Data Loader] Add chain loader documentation

Resources/doc/data-loader/chain.rst

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+
+.. _data-loaders-chain:
+
+Chain Loader
+============
+
+The ``Chain`` data loader doesn't load the image binary itself; instead
+it allows for loading the image binary using any number of other
+configured data loaders. For example, if you configured both a
+:ref:`filesystem <data-loaders-filesystem>` and
+:ref:`flysystem <data-loaders-flysystem>` data loader, this loader can
+be defined to load from both in a defined order, returning the image
+binary from the first that responds.
+
+.. tip::
+
+    This loader iterates over the data loaders in the order they are
+    configured in the chain definition, returning an image binary from
+    the first loader that supports the passed file path. This means if
+    a file exists in more than one loader, the file will be returned
+    using the first one defined in your configuration file for this
+    chain loader.
+
+
+
+Configuration
+-------------
+
+As this loader leverages any number of other configured loaders, its
+configuration is relatively simple; it supports only a ``loaders``
+option that accepts an array of other configured loader names:
+
+.. code-block:: yaml
+
+    # app/config/config.yml
+
+    liip_imagine:
+        loaders:
+            foo:
+                filesystem:
+                    # configure filesystem loader
+
+            bar:
+                flysystem:
+                    # configure flysystem loader
+
+            baz:
+                stream:
+                    # configure stream loader
+
+            qux:
+                chain:
+                    # use the "foo", "bar", and "baz" loaders
+                    loaders:
+                        - foo
+                        - bar
+                        - baz