Update readme

README.md

@@ -1,3 +1,83 @@
 # UserInterfaceVariations
 
-A description of this package.
+Allows defining user interface variations depending on `UIUserInterfaceSizeClass`. It is a substitute of functionality provided by *Xcode Interface Builder*.
+
+## Example usages
+
+Deactivate `NSLayoutConstraint` for compact horizontal size class:
+
+```swift
+addVariation(for: constraint,
+             property: \.isActive,
+             sizeClassDimension: .horizontal,
+             whenCompact: false)
+```
+
+Control `UILabel` `text` in every size class configuration:
+
+```swift
+addVariations([
+    UIVariation(object: label,
+                keyPath: \.text,
+                value: "w:C h:C",
+                horizontalSizeClass: .compact,
+                verticalSizeClass: .compact),
+    UIVariation(object: label,
+                keyPath: \.text,
+                value: "w:C h:R",
+                horizontalSizeClass: .compact,
+                verticalSizeClass: .regular),
+    UIVariation(object: label,
+                keyPath: \.text,
+                value: "w:R h:C",
+                horizontalSizeClass: .regular,
+                verticalSizeClass: .compact),
+    UIVariation(object: label,
+                keyPath: \.text,
+                value: "w:R h:R",
+                horizontalSizeClass: .regular,
+                verticalSizeClass: .regular)
+])
+```
+
+Make only horizontal class matter:
+
+```swift
+addVariations([
+    UIVariation(object: label,
+                keyPath: \.text,
+                value: "w:C h:Any",
+                horizontalSizeClass: .compact,
+                verticalSizeClass: nil),
+    UIVariation(object: label,
+                keyPath: \.text,
+                value: "w:R h:Any",
+                horizontalSizeClass: .regular,
+                verticalSizeClass: nil),
+])
+```
+
+Assign new value to variation:
+
+```swift
+let variation = UIVariation(object: view,
+                            keyPath: \.backgroundColor,
+                            value: .white,
+                            horizontalSizeClass: .compact,
+                            verticalSizeClass: nil)
+variation.value = .black
+```
+
+## Integration
+
+Declare `UIVariationEnvironment` conformance on an object already conforming to `UITraitEnvironment` (e.g. `UIViewController`, `UIView`) and override following method.
+
+```swift
+class ViewController: UIViewController, UIVariationEnvironment {
+    
+    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
+        super.traitCollectionDidChange(previousTraitCollection)
+        activateVariationsMatchingTraitEnvironment()
+    }
+}
+```