Added Readme about Legacy Platforms

LEGACY_PLATFORMS.md

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+SnapKit supports legacy platforms but only through manual integration of the code:
+
+Legacy Platforms are:
+
+* iOS 7.0
+* OS X 10.9
+
+You must integrate SnapKit manually by dragging the source code into your project and in your project you must add `-DSNAPKIT_DEPLOYMENT_LEGACY` to `OTHER_SWIFT_FLAGS` in your target's build settings.
+
+Unfortunately there is no other way to ensure that iOS 8.0/9.0 targets have no warnings and iOS 7.0 still works.