Zephyr: Use editable mode in readme.

Signed-off-by: Zingo Andersen <Zingo.Andersen@arm.com>
Change-Id: Ia27e1028a8bf7943551a2ded656937f8503bfbe6

Author: zingo

zephyr/README.md

@@ -77,7 +77,7 @@ Setup ExecuTorch
 cd modules/lib/executorch/
 git submodule sync
 git submodule update --init --recursive
-./install_executorch.sh
+./install_executorch.sh --editable
 cd ../../..
 ```