update README.md

Author: p-x9

README.md

@@ -1,3 +1,41 @@
 # SCFNotification
 
-A description of this package.
+Swift wrapper of `CFNotificationCenter`.
+No more tedious type conversions using pointers.
+
+## Usage
+import
+```swift
+import SCFNotification
+```
+
+addObserver
+```swift
+SCFNotificationCenter
+            .addObserver(center: .local,
+                         observer: self,
+                         name: .init("local.notification" as CFString),
+                         suspensionBehavior: .deliverImmediately) { center, `self`, name, object, userInfo in
+                print(center, name, object, userInfo)
+            }
+```
+
+postNotification
+```swift
+SCFNotificationCenter
+            .postNotification(center: .local,
+                              name: .init("local.notification" as CFString),
+                              userInfo: [:] as CFDictionary,
+                              deliverImmediately: true
+            )
+
+```
+
+removeObserver
+```swift
+SCFNotificationCenter
+            .removeObserver(center: .local,
+                            observer: self,
+                            name: .init("local.notification" as CFString)
+            )
+```