rm: move help strings to markdown file

[jhowww]

#4368

rm -h outputs the following.

$ ./target/debug/coreutils rm -h
Remove (unlink) the FILE(s)

Usage: ./target/debug/coreutils rm [OPTION]... FILE...

Arguments:
  [files]...  

Options:
  -f, --force               ignore nonexistent files and arguments, never prompt
  -i                        prompt before every removal
  -I                        prompt once before removing more than three files, or when removing recursively. Less      
                            intrusive than -i, while still giving some protection against most mistakes
      --interactive <WHEN>  prompt according to WHEN: never, once (-I), or always (-i). Without WHEN, prompts always   
      --one-file-system     when removing a hierarchy recursively, skip any directory that is on a file system
                            different from that of the corresponding command line argument (NOT IMPLEMENTED)
      --no-preserve-root    do not treat '/' specially
      --preserve-root       do not remove '/' (default)
  -r, --recursive           remove directories and their contents recursively [short aliases: R]
  -d, --dir                 remove empty directories
  -v, --verbose             explain what is being done
  -h, --help                Print help information
  -V, --version             Print version information

By default, rm does not remove directories.  Use the --recursive (-r or -R)
option to remove each listed directory, too, along with all of its contents

To remove a file whose name starts with a '-', for example '-foo',
use one of these commands:
rm -- -foo

rm ./-foo

Note that if you use rm to remove a file, it might be possible to recover
some of its contents, given sufficient expertise and/or time.  For greater
assurance that the contents are truly unrecoverable, consider using shred.