fix unresolved package requirement to install Humble on Ubuntu 20.04

source/Installation/Alternatives/Ubuntu-Development-Setup.rst

@@ -46,6 +46,51 @@ Add the ROS 2 apt repository
 Install development tools and ROS tools
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+Ubuntu Linux 20.04 only
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: bash
+
+   sudo apt update && sudo apt install -y \
+      build-essential \
+      cmake \
+      git \
+      libbullet-dev \
+      python3-colcon-common-extensions \
+      python3-flake8 \
+      python3-pip \
+      python3-pytest-cov \
+      python3-rosdep \
+      python3-setuptools \
+      python3-vcstool \
+      wget
+      # install some pip packages needed for testing
+      python3 -m pip install -U \
+      argcomplete \
+      flake8-blind-except \
+      flake8-builtins \
+      flake8-class-newline \
+      flake8-comprehensions \
+      flake8-deprecated \
+      flake8-docstrings \
+      flake8-import-order \
+      flake8-quotes \
+      pytest-repeat \
+      pytest-rerunfailures \
+      pytest
+   # install Fast-RTPS dependencies
+   sudo apt install --no-install-recommends -y \
+      libasio-dev \
+      libtinyxml2-dev
+   # install Cyclone DDS dependencies
+   sudo apt install --no-install-recommends -y \
+      libcunit1-dev
+
+
+
+Others
+~~~~~~
+
 .. code-block:: bash
 
    sudo apt update && sudo apt install -y \