Add Markdown syntax check exception MD014

The rule "MD014 - Dollar signs used before commands without showing output" is not beneficial for the readability of README.md files. Adding a dollar sign, even if the output is not displayed, helps understand that the line refers to a command line that can be executed in the terminal. For instance, the ROS wiki pages precede all commands with a dollar sign (see for example Navigating the ROS Filesystem for a specific example). This way, you can easily understand if the snippet corresponds to a command or corresponds to something else like the content of a file.