build: convert README to markdown

Also add SPDX license identifier and a copyright notice.

README.md

@@ -1,9 +1,17 @@
-ModemManager.
+<!--
+SPDX-License-Identifier: GPL-2.0-or-later
+
+Copyright (C) 2008 Tambet Ingo  <tambet@gmail.com>
+-->
+
+# ModemManager
+
 ModemManager provides a unified high level API for communicating with mobile
 broadband modems, regardless of the protocol used to communicate with the
 actual device (Generic AT, vendor-specific AT, QCDM, QMI, MBIM...).
 
-Using.
+## Using
+
 ModemManager is a system daemon and is not meant to be used directly from
 the command line. However, since it provides a DBus API, it is possible to use
 'dbus-send' commands or the new 'mmcli' command line interface to control it
@@ -11,7 +19,8 @@ from the terminal. The devices are queried from udev and automatically updated
 based on hardware events, although a manual re-scan can also be requested to
 look for RS232 modems.
 
-Implementation.
+## Implementation
+
 ModemManager is a DBus system bus activated service (meaning it's started
 automatically when a request arrives). It is written in C, using glib and gio.
 Several GInterfaces specify different features that the modems support,
@@ -20,7 +29,8 @@ operations for 3GPP (GSM, UMTS, LTE) or CDMA (CDMA1x, EV-DO) modems. If a given
 feature is not available in the modem, the specific interface will not be
 exported in DBus.
 
-Plugins.
+## Plugins
+
 Plugins are loaded on startup, and must implement the MMPlugin interface. It
 consists of a couple of methods which tell the daemon whether the plugin
 supports a port and to create custom MMBroadbandModem implementations. It most
@@ -31,14 +41,17 @@ used as an example for writing new plugins. Writing new plugins is highly
 encouraged! The plugin API is open for changes, so if you're writing a plugin
 and need to add or change some public method, feel free to suggest it!
 
-License.
+## License
+
 The ModemManager and mmcli binaries are both GPLv2+.
 The libmm-glib library is LGPLv2+.
 
-Code of Conduct.
+## Code of Conduct
+
 Please note that this project is released with a Contributor Code of Conduct.
 By participating in this project you agree to abide by its terms, which you can
 find in the following link:
-  https://www.freedesktop.org/wiki/CodeOfConduct
+https://www.freedesktop.org/wiki/CodeOfConduct
+
 CoC issues may be raised to the project maintainers at the following address:
-  modemmanager-devel-owner@lists.freedesktop.org
+modemmanager-devel-owner@lists.freedesktop.org