Add section for "Archiving and Digital Preservation," with four tools.

This commit adds a new section, "Archiving and Digital Preservation,"
which contains four specialty Web-based applications primarily designed
to service the needs of professional archivists with free software.
These are distinct from simple Content Management Systems (CMS) tools in
that they feature industry-specific requirements for digital
preservation techniques, museum and archives semantic/ontological
XML-based standards (such as Encoded Archival Description, or EAD) atop
the usual CMS infrastructures such as Web document publishing and so on.

README.md

@@ -11,6 +11,7 @@ See [Contributing](.github/CONTRIBUTING.md).
 --------------------
 
 - [Analytics](#analytics)
+- [Archiving and Digital Preservation (DP)](#archiving-and-digital-preservation-dp)
 - [Automation](#automation)
 - [Blogging Platforms](#blogging-platforms)
 - [Bookmarks and Link Sharing](#bookmarks-and-link-sharing)
@@ -116,6 +117,17 @@ _Business Intelligence_
 - [Redash](http://redash.io) - connect to over 18 types of databases (SQL and "NoSQL"), query your data, visualize it and create dashboards. Everything has a URL that can be shared. Slack and HipChat integration. ([Demo](https://demo.redash.io), [Source Code](https://github.com/getredash/redash)) `BSD-2-Clause` `Python`
 - [Superset](http://superset.apache.org/) - Modern, enterprise-ready business intelligence web application. ([Source Code](https://github.com/apache/incubator-superset)) `Apache-2.0` `Python`
 
+## Archiving and Digital Preservation
+
+**[`^        back to top        ^`](#)**
+
+Some [Content Management System](#content-management-systems-cms) solutions also feature archiving and digital preservation.
+
+- [Access to Memory (AtoM)](https://www.accesstomemory.org/) - Web-based, open source application for standards-based archival description and access in a multilingual, multi-repository environment. ([Demo](https://demo.accesstomemory.org/), [Source Code](https://github.com/artefactual/atom)) `PHP` `AGPL-3.0-only`
+- [Archivematica](https://www.archivematica.org/) - Mature digital preservation system designed to maintain standards-based, long-term access to collections of digital objects. ([Demo](http://sandbox.archivematica.org/administration/accounts/login/), [Source Code](https://github.com/artefactual/archivematica)) `Python` `AGPL-3.0-only`
+- [ArchivesSpace](https://archivesspace.org/) - Archives information management application for managing and providing Web access to archives, manuscripts and digital objects. ([Demo](https://archivesspace.org/application/demo/), [Source Code](https://github.com/archivesspace/archivesspace)) `Ruby` `ECL-2.0`
+- [Collective Access: Providence](http://collectiveaccess.org/) - Highly configurable Web-based framework for management, description, and discovery of digital and physical collections supporting a variety of metadata standards, data types, and media formats. ([Source Code](https://github.com/collectiveaccess/providence)) `PHP` `GPL-3.0-only`
+
 ## Automation
 
 **[`^        back to top        ^`](#)**
@@ -1287,6 +1299,7 @@ See also [Documentation Generators](#documentation-generators), [Wikimatrix](htt
 - `⚠ ` - Depends on a third party network service
 - `AAL` - [Attribution Assurance License](https://opensource.org/licenses/AAL)
 - `AGPL-3.0` - [GNU Affero General Public License 3.0](https://www.gnu.org/licenses/agpl-3.0)
+- `AGPL-3.0-only` - [GNU Affero General Public License 3.0 only](https://spdx.org/licenses/AGPL-3.0-only.html)
 - `Apache-2.0` - [Apache, Version 2.0](http://www.apache.org/licenses/)
 - `APSL-2.0` - [Apple Public Source License, Version 2.0](https://opensource.org/licenses/APSL-2.0)
 - `Artistic-2.0` - [Artistic License Version 2.0](http://opensource.org/licenses/Artistic-2.0)
@@ -1307,6 +1320,7 @@ See also [Documentation Generators](#documentation-generators), [Wikimatrix](htt
 - `GPL-1.0` - [GNU General Public License](https://www.gnu.org/licenses/gpl-1.0)
 - `GPL-2.0` - [GNU General Public License 2.0](https://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html)
 - `GPL-3.0` - [GNU General Public License 3.0](http://www.gnu.org/licenses/gpl-3.0.en.html)
+- `GPL-3.0-only` - [GNU General Public License 3.0](https://spdx.org/licenses/GPL-3.0-only.html)
 - `IPL-1.0` - [IBM Public License](https://opensource.org/licenses/IPL-1.0)
 - `ISC` - [Internet Systems Consortium License](https://www.isc.org/downloads/software-support-policy/isc-license/)
 - `LGPL-2.1` - [Lesser General Public License 2.1](http://opensource.org/licenses/LGPL-2.1)