runcon: move help strings to markdown file

[m11o]

runcon --help outputs follow.

$ ./target/debug/runcon --help
Run command with specified security context.

Usage: ./target/debug/runcon [CONTEXT COMMAND [ARG...]]
       ./target/debug/runcon [-c] [-u USER] [-r ROLE] [-t TYPE] [-l RANGE] COMMAND [ARG...]";

Arguments:
  [ARG]...  

Options:
  -c, --compute        Compute process transition context before modifying.
  -u, --user <USER>    Set user USER in the target security context.
  -r, --role <ROLE>    Set role ROLE in the target security context.
  -t, --type <TYPE>    Set type TYPE in the target security context.
  -l, --range <RANGE>  Set range RANGE in the target security context.
  -h, --help           Print help
  -V, --version        Print version

Run COMMAND with completely-specified CONTEXT, or with current or transitioned security context modified by one or more
of LEVEL, ROLE, TYPE, and USER.

If none of --compute, --type, --user, --role or --range is specified, then the first argument is used as the complete
context.

Note that only carefully-chosen contexts are likely to successfully run.

With neither CONTEXT nor COMMAND are specified, then this prints the current security context.