Document process for adding a new feature

[haz]

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.