Improve docs for the defaults key

Closes #59

Sources/Defaults/Defaults.swift

@@ -46,7 +46,9 @@ public enum Defaults {
 		/**
 		Create a defaults key.
 
-		The `default` parameter can be left out if the `Value` type is an optional.
+		- Parameter key: The key must be ASCII, not start with `@`, and cannot contain a dot (`.`).
+
+		The `default` parameter should not be used if the `Value` type is an optional.
 		*/
 		public init(_ key: String, default defaultValue: Value, suite: UserDefaults = .standard) {
 			self.defaultValue = defaultValue
@@ -86,6 +88,11 @@ extension Defaults {
 }
 
 extension Defaults.Key {
+	/**
+	Create a defaults key.
+
+	- Parameter key: The key must be ASCII, not start with `@`, and cannot contain a dot (`.`).
+	*/
 	public convenience init<T>(_ key: String, suite: UserDefaults = .standard) where Value == T? {
 		self.init(key, default: nil, suite: suite)
 	}

readme.md

@@ -80,7 +80,7 @@ If a type conforms to both `NSSecureCoding` and `Codable`, then `Codable` will b
 
 You declare the defaults keys upfront with type and default value.
 
-**Do not use a dot in the key name, because of [this bug](https://github.com/sindresorhus/Defaults/issues/59).**
+**The key name must be ASCII, not start with `@`, and cannot contain a dot (`.`).**
 
 ```swift
 import Cocoa