Add annotator.ext.document documentation

Author: nickstenning

doc/modules/document.rst

@@ -0,0 +1,17 @@
+``annotator.ext.document``
+==========================
+
+This module collects metadata from the loaded page and adds it to new
+annotations in the ``document`` field. Types of metadata collected at present
+include:
+
+- Facebook Open Graph tags
+- Twitter card tags
+- Highwire tags
+- Dublin Core tags
+- PRISM tags
+- Eprints tags
+- Favicon
+- Page title
+- ``rel=alternate``, ``rel=shortlink``, ``rel=bookmark``, and ``rel=canonical``
+  link tags