Update engine README for Params (#7600)
Update engine README for Params (#7600)

### Problem

The engine `README.md` had not been updated for the merge of "subjects" and `Variants` into `Params` (#6170).

### Solution

Overhaul the explanations to refer to `Params`, and expand on how `Params` are propagated from dependents to dependencies.

Additionally, simplify and document the codepath in rule graph construction that selects which `@rule` dependencies to use.
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