Add Zephyr manifest instructions

Zephyr's manifest allows to easily clone the right version of zephyr and
an OpenRTX repo, greatly simplifying OpenRTX build instructions for the
T-TWR Plus.

Author: n1zzo

compiling.md

@@ -106,14 +106,12 @@ You need to install the following packages (their names might vary according to
 
 [Dependency intructions for Fedora and other distros:](https://docs.zephyrproject.org/latest/develop/getting_started/installation_linux.html)
 
-Initialize the Zephyr SDK in a path of your choice:
+Clone a fresh copy of the OpenRTX repo, the Zephyr and associated repos will be cloned in the parent directory of the OpenRTX repo.
 
 ```
-pushd ZEPHYR_PATH
-west init .
-git checkout b6095a878c7e
+git clone git@github.com:OpenRTX/OpenRTX.git; cd OpenRTX
+west init -l --mf ./west.yml; pushd ..
 west update
-west blobs fetch hal_espressif
 source zephyr/zephyr-env.sh
 popd
 ```