clarify license and use SPDX ids

This follows up rpm-software-management/rpm#2511

about.md

@@ -7,7 +7,7 @@ title: rpm.org - About
 
 The RPM Package Manager (RPM) is a powerful command line driven package management system capable of installing, uninstalling, verifying, querying, and updating computer software packages. Each software package consists of an archive of files along with information about the package like its version, a description, and the like. There is also a library API, permitting advanced developers to manage such transactions from programming languages such as C or Python.
 
-RPM is [free software](https://www.gnu.org/philosophy/free-sw.html), released under the [GNU GPL](https://www.fsf.org/licensing/licenses/gpl.html).
+RPM is [free software](https://www.gnu.org/philosophy/free-sw.html), released under the [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html). All of the source code in the `lib` and `rpmio` subdirectories are licensed under [GPL-2.0-or-later](https://spdx.org/licenses/GPL-2.0-or-later.html) OR [LGPL-2.1-or-later](https://spdx.org/licenses/LGPL-2.1-or-later.html).
 
 RPM is a core component of many Linux distributions, such as [Red Hat Enterprise Linux](https://www.redhat.com/rhel/), the [Fedora Project](https://fedoraproject.org/), [SUSE Linux Enterprise](https://www.suse.com/), [openSUSE](https://opensuse.org/), [CentOS](https://centos.org/), [Tizen](https://www.tizen.org/), [Mageia](https://mageia.org/), [CBL-Mariner](https://github.com/microsoft/CBL-Mariner) and many others.