Add clone instructions to readme (backport #1474) (#1475)

* Add clone instructions to readme (#1474)

Signed-off-by: Antón Casas <antoncasas@eprosima.com>
(cherry picked from commit 478e51b)

* Specify branch

---------

Signed-off-by: Antón Casas <antoncasas@eprosima.com>
Co-authored-by: Antón Casas <antoncasas@eprosima.com>

Author: mergify[bot]

README.md

@@ -35,9 +35,10 @@ source ~/.bashrc
 
 ### 2. Compile Example
 
-Once the Pico SDK is ready, compile the example:
+Once the Pico SDK is ready, clone this repository and compile the example:
 
 ```bash
+git clone -b rolling https://github.com/micro-ROS/micro_ros_raspberrypi_pico_sdk
 cd micro_ros_raspberrypi_pico_sdk
 mkdir build
 cd build