pinky: move help strings to markdown file

[m11o]

issue: #4368

pinky --help outputs the following.

./target/debug/pinky --help
Lightweight finger

Usage: ./target/debug/pinky [OPTION]... [USER]...

Arguments:
  [user]...  

Options:
  -l             produce long format output for the specified USERs
  -b             omit the user's home directory and shell in long format
  -h             omit the user's project file in long format
  -p             omit the user's plan file in long format
  -s             do short format output, this is the default
  -f             omit the line of column headings in short format
  -w             omit the user's full name in short format
  -i             omit the user's full name and remote host in short format
  -q             omit the user's full name, remote host and idle time in short format
      --help     Print help information
  -V, --version  Print version

A lightweight 'finger' program;  print user information.
The utmp file will be /var/run/utmpx.

[cakebaker]

Thanks for your PR :)