Crate actions can now define required prerequisites.
Update mods (only d2k and RA affected) crate actions to take tech levels and prerequisites into account.
Added YamlException to GiveUnitCrateAction if no units are defined.
Make ra light tanks be able to crush infantry to improve usefulness
and be consistent with the rest of the ra tanks and ra1's light tank
Fix legacy map importer. Fixes #6143.
Let ra apc and minelayers crush infantry too
Update CONTRIBUTING.md
Merge pull request #6212 from pchote/mapimport

Fix legacy map importer.
Merge pull request #6213 from ScottNZ/balance

Update crushing values for ra vehicles
Merge pull request #6189 from UberWaffe/6066_CratePrerequisites

6066 Crate Prerequisites
TechTree changes
Test