build+ci: switch to python3

Also in this commit:
 * Sync CI scripts with other projects
 * Sync sphinx_build waf tool with ndn-cxx
 * Modernize docs/conf.py
 * Replace redmine_issue extension with sphinx.ext.extlinks
 * Fix typo in docs/doxygen.conf.in
 * Improve README.md

Refs: #5095
Change-Id: Iee05500f907442dd7cb3e41bd613beef7461dab4
diff --git a/README.md b/README.md
index 99cd7af..07e2330 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,22 @@
-# NAC: Named-Based Access Control
+# NAC: Named-Based Access Control Library for NDN
 
+![Language](https://img.shields.io/badge/C%2B%2B-14-blue.svg)
 [![Build Status](https://travis-ci.org/named-data/name-based-access-control.svg?branch=new)](https://travis-ci.org/named-data/name-based-access-control)
 
-Please submit any bugs or issues to the NAC issue tracker:
-https://redmine.named-data.net/projects/nac/issues
+## Reporting bugs
+
+Please submit any bug reports or feature requests to the
+[NAC issue tracker](https://redmine.named-data.net/projects/nac/issues).
+
+## Contributing
+
+We greatly appreciate contributions to the NAC code base, provided that they are
+licensed under the LGPL 3.0+ or a compatible license (see below).
+If you are new to the NDN software community, please read the
+[Contributor's Guide](https://github.com/named-data/.github/blob/master/CONTRIBUTING.md)
+to get started.
+
+## License
+
+NAC is an open source project licensed under the LGPL version 3.
+See [`COPYING.md`](COPYING.md) for more information.