docs: Include an example for using DecodeMulti in the Readme (#69)

* Include an example for using DecodeMulti in the Readme

* Put warning in as code comment

README.md

@@ -77,6 +77,64 @@ registered first using gob.Register(). For basic types this is not needed;
 it works out of the box. An optional JSON encoder that uses `encoding/json` is
 available for types compatible with JSON.
 
+### Key Rotation
+Rotating keys is an important part of any security strategy. The `EncodeMulti` and
+`DecodeMulti` functions allow for multiple keys to be rotated in and out.
+For example, let's take a system that stores keys in a map:
+
+```go
+// keys stored in a map will not be persisted between restarts
+// a more persistent storage should be considered for production applications.
+var cookies = map[string]*securecookie.SecureCookie{
+	"previous": securecookie.New(
+		securecookie.GenerateRandomKey(64),
+		securecookie.GenerateRandomKey(32),
+	),
+	"current": securecookie.New(
+		securecookie.GenerateRandomKey(64),
+		securecookie.GenerateRandomKey(32),
+	),
+}
+```
+
+Using the current key to encode new cookies:
+```go
+func SetCookieHandler(w http.ResponseWriter, r *http.Request) {
+	value := map[string]string{
+		"foo": "bar",
+	}
+	if encoded, err := securecookie.EncodeMulti("cookie-name", value, cookies["current"]); err == nil {
+		cookie := &http.Cookie{
+			Name:  "cookie-name",
+			Value: encoded,
+			Path:  "/",
+		}
+		http.SetCookie(w, cookie)
+	}
+}
+```
+
+Later, decode cookies. Check against all valid keys:
+```go
+func ReadCookieHandler(w http.ResponseWriter, r *http.Request) {
+	if cookie, err := r.Cookie("cookie-name"); err == nil {
+		value := make(map[string]string)
+		err = securecookie.DecodeMulti("cookie-name", cookie.Value, &value, cookies["current"], cookies["previous"])
+		if err == nil {
+			fmt.Fprintf(w, "The value of foo is %q", value["foo"])
+		}
+	}
+}
+```
+
+Rotate the keys. This strategy allows previously issued cookies to be valid until the next rotation:
+```go
+func Rotate(newCookie *securecookie.SecureCookie) {
+	cookies["previous"] = cookies["current"]
+	cookies["current"] = newCookie
+}
+```
+
 ## License
 
 BSD licensed. See the LICENSE file for details.