Add more documentation about what filter options are available

[HaSistrunk]

Is your feature request related to a problem? Please describe.

There isn't a lot of documentation or explanation about how to find the filter options. There are a few examples of possible filters to use when creating an export, and a statement that: "If you have access to the official Fluxx API documentation, more information is available in the "API Filter Examples" section."

Describe the solution you'd like

1. Clarify in the README that there is no public documentation about what filters are available (if that's true?), and that users will need to locate that documentation via Fluxx.
2. Review the filter examples that are given in the README and determine if there are more examples necessary to cover the primary use cases to limit reliance on API documentation.

Describe alternatives you've considered

Even if we don't make substantive changes to the README, adjusting the "If you have access to" language to something more definitive about the need to reference the official Fluxx API documentation, which is not publicly available, would be useful. Current language is unclear about if/how to access that info.