README: Add instructions to have libsystemd >= 227

Works on F23. There are few dependencies pulled by configure that I did
not put there.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

README.md

@@ -11,6 +11,35 @@ environment:
 
     sudo cp dbus/org.freedesktop.ratbag1.conf /etc/dbus-1/system.d/org.freedesktop.ratbag1.conf
 
+Compiling ratbagd
+-----------------
+
+ratbagd needs systemd >= 227. If you are running Fedora 23, you might find the
+following instruction valuable:
+
+Download systemd v229 (master caused troubles with gcrypt and gpg-error last
+time I tried).
+
+    $ git clone --branch v229 https://github.com/systemd/systemd
+
+
+Configure it with:
+
+    $ ./autogen.sh && ./configure --prefix=/opt/systemd \
+                                  --libdir=/opt/systemd/lib \
+                                  --disable-gcrypt \
+                                  --without-bashcompletiondir \
+                                  --with-rootprefix=/opt/systemd \
+                                  --with-sysvinit-path=/opt/systemd/etc \
+                                  --with-sysvrcnd-path=/opt/systemd/etc/rc.d
+
+Then run make and make install.
+
+In ratbagd, you will need to add the newly installed libsystemd in the
+pkg_config path:
+
+    $ PKG_CONFIG_PATH=/opt/systemd/lib/pkgconfig ./configure
+
 License
 -------