docs(README.md): document exit codes — radicle-native-ci

Signed-off-by: Lars Wirzenius liw@liw.fi

 * `log` --- required, file where native CI should write a log
 ## Exit codes
 The `radicle-native-ci` executable uses the following exit codes:
 * 0 = success, everything went well.
 * 1 = failure, the CI run finished, but failed
 * 2 = error, the CI run could not be finished
 An example of a failure is when the CI run builds software that has a
 syntax error.
 An example of an error is when the adapter configuration file is
 missing.
 ## Packaging
 There is simple, simplistic, rudimentary, personal-use-only packaging