Added list of supported feature requirements.

doc/feature-requirements.md

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+# Feature Requirements
+
+Feature requirements are part of `plasp`’s [output format](output-format.md).
+Currently, `plasp` detects feature requirements only for SAS problems.
+
+`plasp` supports the following feature requirements:
+
+SAS feature | description
+------------|------------
+`actionCosts` | actions have associated costs (see [action costs](output-format.md#action-costs))
+`axiomRules` | immediate actions are used, which are executed as soon as the preconditions are satisfied (see [axiom rules](output-format.md#axiom-rules))
+`conditionalEffects` | some effects of an action may have additional conditions (see [actions](output-format.md#actions))
+
+## Example
+
+The following specifies that the input problem has the two requirements `actionCosts` and `conditionalEffects`.
+
+```prolog
+% requirements
+requiresFeature(actionCosts).
+requiresFeature(conditionalEffects).
+```