[gupnp] Doc: Improve README.md a bit

[Jens Georg <jensgeorg src gnome org>]

commit f6b74234da804fecb17b5ecf2addf68f4d0c58b8
Author: Jens Georg <mail jensge org>
Date:   Sun May 30 13:31:15 2021 +0200

    Doc: Improve README.md a bit

 README.md | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)
---
diff --git a/README.md b/README.md
index f69b5aa..9536432 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,4 @@
-GUPnP
-=====
+# GUPnP
 
 GUPnP is an object-oriented open source framework for creating UPnP devices and
 control points, written in C using GObject and libsoup. The GUPnP API is
@@ -21,4 +20,20 @@ threads. GUPnP is entirely single-threaded (though asynchronous), integrates
 with the GLib main loop, and provides the same set of features as libupnp while
 hiding most of the UPnP internals through an elegant object-oriented design.
 
-GUPnP is free software released under the GNU LGPL.
+GUPnP is free software released under the GNU LGPL version 2.1 or later.
+
+## Building
+
+GUPnP uses the meson build system. To build GUPnP, the simplest variant is
+```
+$ meson setup build
+$ meson compile -C build
+```
+
+There are several options to customize the build, please see (meson_options.txt) for
+a brief overview.
+
+## Developer documentation
+
+The developer documentation is available at https://gnome.pages.gitlab.gnome.org/gupnp/docs/
+