Update README.md to fix humble from source instructions (#431)

Fix the branch name when checking out ros_gz otherwise `rosdep install -r --from-paths src -i -y --rosdistro humble` errors with incompatible versions. Additonally link to #401 (comment) which allows compilation on Ubuntu 20.04 + ROS2 Humble + Ignition Fortress

Signed-off-by: Samuel Seng <[email protected]>

README.md

@@ -83,7 +83,7 @@ Install either [Edifice, Fortress, or Garden](https://gazebosim.org/docs).
 Set the `GZ_VERSION` environment variable to the Gazebo version you'd
 like to compile against. For example:
 
-    export GZ_VERSION=edifice
+    export GZ_VERSION=edifice # IMPORTANT: Replace with correct version
 
 > You only need to set this variable when compiling, not when running.
 
@@ -99,7 +99,7 @@ The following steps are for Linux and OSX.
     cd ~/ws/src
 
     # Download needed software
-    git clone https://github.com/gazebosim/ros_gz.git -b ros2
+    git clone https://github.com/gazebosim/ros_gz.git -b humble
     ```
 
 1. Install dependencies (this may also install Gazebo):
@@ -122,6 +122,22 @@ The following steps are for Linux and OSX.
     colcon build
     ```
 
+    If `colcon build` fails with [this issue](https://github.com/gazebosim/ros_gz/issues/401)
+
+    ```
+    CMake Error at CMakeLists.txt:81 (find_package):
+      By not providing "Findactuator_msgs.cmake" in CMAKE_MODULE_PATH this
+      project has asked CMake to find a package configuration file provided by
+      "actuator_msgs", but CMake did not find one.
+    ```
+
+    ```bash
+    cd src
+    git clone [email protected]:rudislabs/actuator_msgs.git
+    cd ../
+    colcon build
+    ```
+
 ## ROSCon 2022
 
 [![](img/video_img.png)](https://vimeo.com/showcase/9954564/video/767127300)