add note on pkg prefix command

README.md

@@ -41,6 +41,7 @@
    * ```ros2 param list```
    * [colcon](pages/colcon.md) is the build tool.
    * ```ros2 doctor --report``` gives tons of information.
+   * ```ros2 pkg prefix <pkg>``` tells where a package is located.
  * Packaging
    * [Setting bloom/git to always use ssh](https://answers.ros.org/question/234494/diagnosing-issues-with-bloom-github-two-factor-authentication/)
      * ```git config --global url.ssh://[email protected]/.insteadOf https://github.com/```