Add an example about test splitting

[mw-hrastega]

This pull request adds a new command parameter and example to README.md. The parameter and example were previously reviewed by Mark. But please take another look as sanity check.

[mcafaro]

Looks good! We should wait until the select-by-name input is actually in the orb before merging though.

[mw-hrastega]

Looks good! We should wait until the select-by-name input is actually in the orb before merging though.

Good point. @mw-kapilg, could you add the missing parameter? Thanks!

[mw-kapilg]

Thanks Houman for creating this PR again! I'll take care of merging the implementation changes (just needs a final verification of successful pipeline run)