Add section on special bag structures (#354)

I've added a section about alternate configurations for bags. It's a bit
of a punt, as we don't test all of the possible bag configurations, but
I'm not sure how else to communicate that there may be other setup
options out there while also acknowledging that A) Artefactual doesn't
test them all, B) some of them might not work, and C) if they don't work
it doesn't mean it's a bug. So this is what I've landed on.

Connected to archivematica/Issues#669

user-manual/transfer/bags.rst

@@ -8,12 +8,16 @@ Archivematica supports the ingest of materials packaged in accordance with the
 Library of Congress `BagIt`_ specification. Users can ingest both zipped and
 unzipped bags by using the appropriate :ref:`transfer type <transfer-types>`.
 
-*On this page:*
+*On this page*
 
 * :ref:`Bag structure requirements <bag-structure-requirements>`
-* :ref:`Unzipped bags <unzipped-bags>`
-* :ref:`Zipped bags <zipped-bags>`
-* :ref:`Adding metadata to bags <adding-metadata-bags>`
+* :ref:`Configuring bags <configure-bags>`
+
+  * :ref:`Unzipped bags <unzipped-bags>`
+  * :ref:`Zipped bags <zipped-bags>`
+  * :ref:`Adding metadata to bags <adding-metadata-bags>`
+  * :ref:`Other ways to configure bags <other-bag-configs>`
+
 * :ref:`Index and search bag metadata <searching-bags>`
 
 .. _bag-structure-requirements:
@@ -33,10 +37,18 @@ Some issues that can cause bags to inadvertently fail include:
 * Structural issues. See :ref:`Adding metadata to bags
   <metadata-bags>` for more information on how to structure bags.
 
+.. _configure-bags:
+
+Configuring bags
+----------------
+
+Bags can be configured in a variety of ways to achieve specific preservation
+goals.
+
 .. _unzipped-bags:
 
 Unzipped bags
--------------
+^^^^^^^^^^^^^
 
 Unzipped bags can be created by hand or by using a BagIt tool like
 `BagIt-python`_ or `Bagger`_. The bag must comply with the `BagIt`_
@@ -64,7 +76,7 @@ For more information on processing your transfer, see :ref:`process transfer
 .. _zipped-bags:
 
 Zipped bags
------------
+^^^^^^^^^^^
 
 Zipped bags can be created by hand or by using a BagIt tool like `BagIt-python`_
 or `Bagger`_. The bag must comply with the `BagIt`_ specification.
@@ -92,12 +104,31 @@ For more information on processing your transfer, see :ref:`process transfer
 .. _adding-metadata-bags:
 
 Adding descriptive/rights metadata and submission documentation to bags
------------------------------------------------------------------------
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Similar to standard transfers, it is possible to add descriptive and rights
 metadata to unzipped and zipped bag transfers. See :ref:`Adding metadata to bags
 <metadata-bags>` for more information.
 
+.. _other-bag-configs:
+
+Other ways to configure bags
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+There may be other ways to combine standard transfer configurations with bags,
+such as:
+
+* :ref:`Transfers with submission documentation <create-submission>`
+* :ref:`Persistent identifiers <transfer-pids>`
+* :ref:`Transferring material with preservation or access derivatives (manual normalization) <transfer-derivatives>`
+* :ref:`Transferring material with access copies only <transfer-access-copies>`
+* :ref:`Transferring material with service (mezzanine) files <transfer-service-files>`
+
+However, Artefactual does not test all configurations. It is recommended to
+thoroughly test any unique configuration and review the results before running
+your workflow in production. If you have questions about different ways to set
+up bags for transfer, consider a post to the `Archivematica user forum`_.
+
 .. _searching-bags:
 
 Index and search bag metadata
@@ -213,3 +244,4 @@ user enters two dates in ISO date format separated by a colon. For example,
 .. _BagIt: https://tools.ietf.org/html/rfc8493
 .. _BagIt-python: https://github.com/LibraryOfCongress/bagit-python
 .. _Bagger: https://github.com/LibraryOfCongress/bagger
+.. _Archivematica user forum: https://groups.google.com/forum/#!forum/archivematica