Document the "-m plop.collector" functionality.

Document how to use "python -m plop.collector".

Also, specify how to write out collector.stack_counts.
Merge pull request #2 from lorin/readme-add

Document the "-m plop.collector" functionality.
Aligning with pullrequest #3 convention, CWD should be default over /tmp
Merge https://github.com/bdarnell/plop
Clarified documentation