true: move help strings to markdown file

[piotrkwiecinski]

#4368

true --help outputs the following:

target/debug/true --help
Returns true, a successful exit status.

Immediately returns with the exit status `0`, except when invoked with one of the
recognized
options. In those cases it will try to write the help or version text. Any IO error during
this
operation causes the program to return `1` instead.

Usage: true

Options:
      --help     Print help information
      --version  Print version information

[cakebaker]

Thanks!