Merge pull request #375 from saaketp/PackageMetadata-docs

Add note about PackageMetadata protocol and include link to importlib_metadata docs in usage.

docs/using.rst

@@ -23,6 +23,13 @@ By default, package metadata can live on the file system or in zip archives on
 anywhere.
 
 
+.. seealso::
+
+   https://importlib-metadata.readthedocs.io/
+      The documentation for ``importlib_metadata``, which supplies a
+      backport of ``importlib.metadata``.
+
+
 Overview
 ========
 
@@ -162,6 +169,13 @@ all the metadata in a JSON-compatible form per PEP 566::
     >>> wheel_metadata.json['requires_python']
     '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
 
+.. note::
+
+    The actual type of the object returned by ``metadata()`` is an
+    implementation detail and should be accessed only through the interface
+    described by the
+    `PackageMetadata protocol <https://importlib-metadata.readthedocs.io/en/latest/api.html#importlib_metadata.PackageMetadata>`.
+
 
 .. _version: