README.effects: Document exact actions supported by Casus Belli effects
https://www.hostedredmine.com/issues/874200 does just a stop-gag documentation update to make it clear that Casus Belli effects are supported only for some actions, so that ruleset authors won't get nasty surprises. It still leaves it undocumented what the supported actions then are.
S3_1 and S3_0 patches attached. Master patch to do.