Add section to point users to the list of parameter file inputs

[glemieux]

Is your feature request related to a problem? Please describe.
Similar to #45, give users the ability to easily oriented themselves to and reference the full list of parameter file input variables.

Describe the solution you'd like
Simply point to the relevant piece of code for simpilicity. This avoids duplication/mirroring lags.

Describe alternatives you've considered
Longer-term we could create a tool that we could automate pulling the code and updating the fates users guide to allow for viewing through the user's guide itself (i.e. with a table). See #46

Additional context
This is being done in lieu of NGEET/fates-docs#4.