clarify CATKIN_IGNORE may be a file, directory, or (dangling) symlink (…

…#256)

* Update rep-0128.rst

Mention that CATKIN_IGNORE may be any type of file

* Update rep-0128.rst

change "are ignored" to "don't matter" to avoid implying that that's what "IGNORE" is referring to in "CATKIN_IGNORE"

* Specify dangling symlink is okay.

Co-authored-by: Dan Rose <rotu@users.noreply.github.com>

rep-0128.rst

@@ -37,7 +37,7 @@ This is the recommended layout for development is as follows:
           package.xml
         ...
         package_n/
-          CATKIN_IGNORE      --Optionally place this empty marker file to exclude package_n from being processed
+          CATKIN_IGNORE      --Optionally place this marker file to exclude package_n from being processed. Its file type (e.g. regular file, directory) and contents don't matter. It may even be a dangling symlink.
           CMakeLists.txt
           package.xml
       build/                 --BUILD SPACE(this is where build system is invoked, not necessarily within workspace)