Add some more rule rationales

Source/SwiftLintBuiltInRules/Rules/Idiomatic/LegacyNSGeometryFunctionsRule.swift

@@ -8,6 +8,12 @@ struct LegacyNSGeometryFunctionsRule: Rule {
         identifier: "legacy_nsgeometry_functions",
         name: "Legacy NSGeometry Functions",
         description: "Struct extension properties and methods are preferred over legacy functions",
+        rationale: """
+        The CGRect extension properties are a more modern API (and are available on NSRect, which
+        is a typealias for CGRect), and are supported on all Swift platforms.
+
+        The legacy functions are only supported on macOS and Mac Catalyst.
+        """,
         kind: .idiomatic,
         nonTriggeringExamples: [
             Example("rect.width"),

Source/SwiftLintBuiltInRules/Rules/Performance/ContainsOverFilterCountRule.swift

@@ -8,6 +8,10 @@ struct ContainsOverFilterCountRule: Rule {
         identifier: "contains_over_filter_count",
         name: "Contains over Filter Count",
         description: "Prefer `contains` over comparing `filter(where:).count` to 0",
+        rationale: """
+        `filter` always needs to scan the entire collection, whereas `contains` can exit early as
+        soon as a match is found.
+        """,
         kind: .performance,
         nonTriggeringExamples: [">", "==", "!="].flatMap { operation in
             [

Source/SwiftLintBuiltInRules/Rules/Performance/ContainsOverFilterIsEmptyRule.swift

@@ -8,6 +8,10 @@ struct ContainsOverFilterIsEmptyRule: Rule {
         identifier: "contains_over_filter_is_empty",
         name: "Contains over Filter is Empty",
         description: "Prefer `contains` over using `filter(where:).isEmpty`",
+        rationale: """
+        `filter` always needs to scan the entire collection, whereas `contains` can exit early as
+        soon as a match is found.
+        """,
         kind: .performance,
         nonTriggeringExamples: [">", "==", "!="].flatMap { operation in
             [

Source/SwiftLintBuiltInRules/Rules/Performance/EmptyCountRule.swift

@@ -11,6 +11,10 @@ struct EmptyCountRule: Rule {
         identifier: "empty_count",
         name: "Empty Count",
         description: "Prefer checking `isEmpty` over comparing `count` to zero",
+        rationale: """
+        For collections that do not conform to `RandomAccessCollection`, `count` is an O(n) operation,
+        whereas `isEmpty` is O(1).
+        """,
         kind: .performance,
         nonTriggeringExamples: [
             Example("var count = 0"),