Create an appendix on for freqtables

[mbcann01]

Essentially, make the appendix a complete guide to using freqtables and all of its functions.

• Start with the vignettes that are already in the freqtables package.
• Put links to the Appendix in the freqtables package README and in the individual function help documentation.

Or do you just want to make a packagedown page for freqtables and just point readers there? That is probably the more robust option.