Document filter usage

[ptere]

Goal

• As a user of things-cli
• I want to export content from some projects but not others
• so that I can manage that content elsewhere

Suggested approach In order to achieve this goal I suggest to add examples to the Readme showing usage of the project, area, and tag filters.

Additional context The CLI offers these arguments:

  -p FILTER_PROJECT, --filter-project FILTER_PROJECT
                        filter by project
  -a FILTER_AREA, --filter-area FILTER_AREA
                        filter by area
  -t FILTER_TAG, --filtertag FILTER_TAG
                        filter by tag

however I haven't been able to get any data out when using the -p or -a filters. I'm not clear if they require a name or a UUID, but I've tried both and always get empty results.