Document process for adding a new feature

Should have some example code in this repo to add a new feature across the problems in a particular formalism. Possible first examples:

- [ ] Number of lifted actions
- [ ] Number of predicates
- [ ] Number of FD ground operators
- [ ] Number of FD ground variables

The first 2 could be done with the `pddl` library, while the latter 2 could be done by parsing the FastDownward output.