utils/readme.txt: document docker-run script

Signed-off-by: Marcus Hoffmann <buildroot@bubu1.eu>
[yann.morin.1998@free.fr: the default is an interactive shell]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

utils/readme.txt

@@ -18,6 +18,11 @@ check-package
     It checks the .checkpackageignore file if errors should be ignored and
     errors if there's a file listed that doesn't produce an error.
 
+docker-run
+    a script that runs a command (like make check-package) inside the
+    buildroot CI docker container; pass no command to get an interactive
+    shell.
+
 genrandconfig
     a script that generates a random configuration, used by the autobuilders
     (http://autobuild.buildroot.org). It selects a random toolchain from