README.md: update to include the 20.x build info

Signed-off-by: John Crispin <john@phrozen.org>

README.md

@@ -6,7 +6,7 @@ Install build packages:  sudo apt install build-essential libncurses5-dev gawk g
 
 Plus specific for TIP: sudo apt-get install openvswitch-common
 
-# Doing a native build on Linux
+# Doing a native build on Linux (OpenWrt 19.07 based)
 First we need to clone and setup our tree. This will result in an openwrt/.
 ```
 python3 setup.py --setup
@@ -28,6 +28,22 @@ make -j X V=s
 ```
 Builds for different profiles can co-exist in the same tree. Switching is done by simple calling gen_config.py again.
 
+# Doing a native build on Linux (OpenWrt 20.x based)
+First we need to clone and setup our tree. This will result in an openwrt-20.x/.
+```
+python3 setup.py --setup --20x
+```
+Next we need to select the profile and base package selection. This setup will install the feeds, packages and generate the .config file. The available profiles are ap2220, ea8300, ecw5211, ecw5410. It is no longer required to add the wifi profile. Board profiles will now include their specific wifi flavour profile automatically.
+```
+cd openwrt
+./scripts/gen_config.py ap2220 wlan-ap
+```
+Finally we can build the tree.
+```
+make -j X V=s
+```
+Builds for different profiles can co-exist in the same tree. Switching is done by simple calling gen_config.py again.
+
 # Doing a docker build
 
 Start by installing docker.io on your host system and ensuring that you can run an unprivileged container.