add some preview pictures in readme

Author: rainu

README.md

@@ -6,6 +6,10 @@
 
 [Try it out!](https://rainu.github.io/dev-notes/)
 
+![types_1](preview/cards_2.png) ![types_2](preview/cards_1.png) ![types_1](preview/cards_0.png)
+
+![new](preview/new.png) ![backups](preview/backups.png)
+
 ## Features
 * **Serverless** note app (no server or database required)
     * The whole content will be **stored locally**