feat: Add docker pull stats (supertokens#548)

Author: KShivendu

README.md

@@ -5,6 +5,7 @@
 <a href="https://supertokens.io/discord">
 <img src="https://img.shields.io/discord/603466164219281420.svg?logo=discord"
     alt="chat on Discord"></a>
+<span><img src="https://img.shields.io/docker/pulls/supertokens/supertokens-postgresql.svg" alt="Docker pull stats"/></span>
 
 Add **secure login and session management** to your apps. [SDKs available](https://supertokens.com/docs/community/sdks) for popular languages and front-end frameworks e.g. Node.js, Go, Python, React.js, React Native, Vanilla JS, etc.