update parallel docs about printing to stdout (#1052)

Author: charlierudolph

docs/cli.md

@@ -103,6 +103,8 @@ You can run your scenarios in parallel with `--parallel <NUMBER_OF_SLAVES>`. Eac
 * `CUCUMBER_TOTAL_SLAVES` - set to the number of slaves
 * `CUCUMBER_SLAVE_ID` - id for slave ('0', '1', '2', etc)
 
+**Note**: Printing to `stdout` (using `console.log` or other means) will cause an error, because the slave processes communicate with the master process over `stdout`. Instead print to `stderr` (using `console.error` or other means).
+
 ## Profiles
 
 In order to store and reuse commonly used CLI options, you can add a `cucumber.js` file to your project root directory. The file should export an object where the key is the profile name and the value is a string of CLI options. The profile can be applied with `-p <NAME>` or `--profile <NAME>`. This will prepend the profile's CLI options to the ones provided by the command line. Multiple profiles can be specified at a time. If no profile is specified and a profile named `default` exists, it will be applied.