README.rst: discourage to use rauc-native from DEPLOYDIR

In master, with commit 79554c6 ("rauc-native: do not deploy tool
anymore") support for running rauc from DEPLOYDIR was removed.

Let's keep it in kirkstone to not break existing workflows, but
discourage to use it and document the proper oe-run-native approach
instead.

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>

README.rst

@@ -62,11 +62,15 @@ other layers needed. e.g.::
 II. Building RAUC Host Tool
 ===========================
 
-If you only intend to build the RAUC host tool, you may simply run::
+If you intend to build and use RAUC as a host tool from you BSP, e.g. for
+calling ``rauc info`` on your built bundle, simply run::
 
-  bitbake rauc-native
+  bitbake rauc-native -caddto_recipe_sysroot
+  oe-run-native rauc-native rauc info --keyring=/path/to/keyring.pem tmp/deploy/images/<machine>/<bundle-name>.raucb
 
-This will place the rauc binary at ``tmp/deploy/tools/rauc``.
+For kirkstone still supported but not recommended is to run ``bitbake
+rauc-native`` and use the binary from ``tmp/deploy/tools/rauc`` (together with
+the possibly not-matching host libraries).
 
 If you need to execute the ``casync`` host tool manually, you can do this by running::