add rosdep install to Setup instruction for resolving dependencies

[takasehideki]

When I tried to use this awesome repository, I found ament-cmake-vendor-package is needed to build this package.
I tested this behavior by using both iron and rolling images on Docker Official Image for ros, but it seems that this package is not installed by default.

% docker run -it ros:rolling              
# sudo apt update && sudo apt install curl
# curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
# source "$HOME/.cargo/env"
# mkdir -p ws/src && cd ws/src/
# git clone https://github.com/ros2/rmw_zenoh
# cd ..
# colcon build --cmake-args -DCMAKE_BUILD_TYPE=Release
Starting >>> zenoh_c_vendor
--- stderr: zenoh_c_vendor                         
CMake Error at CMakeLists.txt:11 (find_package):
  By not providing "Findament_cmake_vendor_package.cmake" in
  CMAKE_MODULE_PATH this project has asked CMake to find a package
  configuration file provided by "ament_cmake_vendor_package", but CMake did
  not find one.

  Could not find a package configuration file provided by
  "ament_cmake_vendor_package" with any of the following names:

    ament_cmake_vendor_packageConfig.cmake
    ament_cmake_vendor_package-config.cmake

  Add the installation prefix of "ament_cmake_vendor_package" to
  CMAKE_PREFIX_PATH or set "ament_cmake_vendor_package_DIR" to a directory
  containing one of the above files.  If "ament_cmake_vendor_package"
  provides a separate development package or SDK, be sure it has been
  installed.


---
Failed   <<< zenoh_c_vendor [0.11s, exited with code 1]

Summary: 0 packages finished [0.41s]
  1 package failed: zenoh_c_vendor
  1 package had stderr output: zenoh_c_vendor
  1 package not processed

Of course, those familiar with ROS will quickly realize that they can just operate apt install ros-<DISTRO>-ament-cmake-vendor-package to fix this issue. But there are others who don't. I think it would be not good to waste time before trying out this wonderful package. Also, I understand that it is also possible that this operation may not be necessary in the future.
However, I want to suggest adding a note about this solution for now.

[Yadunund]

Thanks for the PR to improve the build experience. I think right way to resolve this issue (and other dependency related issues that may arise in the future), is to add a rosdep install step to the Setup instructions.

mkdir ~/ws_rmw_zenoh/src -p && cd ~/ws_rmw_zenoh/src
git clone https://github.com/ros2/rmw_zenoh.git
cd ~/ws_rmw_zenoh
+ rosdep install --from-paths src --ignore-src --rosdistro <DISTRO> -y # replace <DISTRO> with ROS 2 distro of choice
source /opt/ros/<DISTRO>/setup.bash # replace <DISTRO> with ROS 2 distro of choice
colcon build --cmake-args -DCMAKE_BUILD_TYPE=Release

This way rosdep can help install other dependency packages too that may not be present on the user's system.

Do you mind updating this PR to include that line?

[takasehideki]

@Yadunund Thank you for reviewing!
Yes, that way is smarter. I added the commit to add the instruction and changed the PR title.

[Yadunund]

Thanks for the quick change.