readme: Added section with build instructions

(cherry picked from commit def868e2)

README.md

@@ -2,6 +2,19 @@
 
 This layer can be used to create an Image for SECO North devices without GPLv3 software.
 
+## Build Instructions
+
+The image build without GPLv3 is based on an additional distro:
+
+``MACHINE=imx8mpguf DISTRO=guf-wayland-no-gplv3 BBLAYERS=bblayers_imx8.conf source ./setup-environment build``
+
+The layer provides a bbclass, that can be used to create new images without GPLv3.
+There is also a default image, that has similar functionality as the SECO North default image (guf-image):
+
+``bitbake seco-image-nogplv3``
+
+For a full example of all build steps, please see the README in the machine layer (https://git.seco.com/seco-ne/yocto/layers/meta-guf-machine).
+
 ## Limitations
 
 A lot of Linux base-utils are released under the GPLv3 license. Because of this, most of these utils are replaced by their BusyBox variant.