Manual build instructions missing dependencies

[MiguelCompany]

When following the instructions here on a fresh install of Ubuntu Focal, I had to install some dependencies that were not mentioned on the instructions.

First step (Build ROS 2 Master) went smoothly.
For the second step (Build Navigation2 Dependencies) I had to add the following dependencies:

# install boost
sudo apt install libboost-all-dev
# install gazeebo
curl -sSL http://get.gazebosim.org | sh

For the third step (Build Navigation2 Master) I had to add the following dependencies:

sudo apt install libgraphicsmagick++1-dev

Afterwards, I realized there is a troubleshooting guide, and repeated the process adding the proper call to rosdep before doing colcon build

Perhaps the best solution would be to add the rosdep step to the instructions.

[SteveMacenski]

Did you use rosdep? Gazebo, Magick, and gazebo are all rosdep entries in the package.xmls (https://github.com/ros/rosdistro/blob/master/rosdep/base.yaml)

Got it - I didn't know that was missing, good catch. Can you submit a PR to add the following before building the dependencies and nav2 workspaces?

rosdep install -y -r -q --from-paths src --ignore-src --rosdistro <ros2-distro>

[MiguelCompany]

Can you submit a PR to add the following before building the dependencies and nav2 workspaces?

Done on #42