Readme updated for custom storage (#68)

README.md

@@ -179,3 +179,6 @@ After webhook config is passed to setupFdk whenever extension is launched to any
 > Any update to webhook config will not automatically update subscriber data on Fynd Platform for a company until extension is opened atleast once after the update. 
 
 Other way to update webhook config manually for a company is to call `syncEvents` function of webhookRegistery.   
+
+# [Custom storage class](/express/storage/README.md)
+The FDK Extension JavaScript library provides built-in support for SQLite, Redis and in-memory storage options as default choices for session data storage. However, if you require a different storage option, this readme will guide you through the process of implementing a custom storage class.

express/storage/README.md

@@ -0,0 +1,88 @@
+#### How to create a custom storage class?
+Custom storage classes allow you to implement extension storage in your preferred database. To achieve this, you are required to create a custom storage class by extending the base storage class provided by the fdk extension JavaScript library and implementing the mandatory methods according to your chosen database.
+
+```javascript
+const BaseStorage = require('fdk-extension-javascript');
+
+class MyCustomStorage extends BaseStorage {
+    constructor(client, prefixKey) {
+        super(prefixKey);
+        this.client = client;
+    }
+
+    // All of the below methods must be implemented per your chosen database.
+
+    async get(key) {
+        // Implementation of a get method
+    }
+
+    async set(key, value) {
+        // Implementation of a set method
+    }
+
+    async del(key) {
+        // Implementation of a del method
+    }
+
+    async setex(key, value, ttl) {
+        // Implementation of a setex method
+    }
+
+    async hget(key, hashKey) {
+        // Implementation of a hget method
+    }
+
+    async hset(key, hashKey, value) {
+        // Implementation of a hset method
+    }
+
+    async hgetall(key) {
+        // Implementation of a hgetall method
+    }
+}
+```
+
+Example implementation of Redis Storage class
+
+```javascript
+'use strict';
+
+const BaseStorage = require('fdk-extension-javascript');
+
+class RedisStorage extends BaseStorage {
+    constructor(client, prefixKey) {
+        super(prefixKey);
+        this.client = client;
+    }
+
+    async get(key) {
+        return await this.client.get(this.prefixKey + key);
+    }
+
+    async set(key, value) {
+        return await this.client.set(this.prefixKey + key, value);
+    }
+
+    async setex(key, value, ttl) {
+        return await this.client.setex(this.prefixKey + key, ttl, value);
+    }
+
+    async del(key) {
+        this.client.del(this.prefixKey + key);
+    }
+
+    async hget(key, hashKey) {
+        return await this.client.hget(this.prefixKey + key, hashKey);
+    }
+
+    async hset(key, hashKey, value) {
+        return await this.client.hset(this.prefixKey + key, hashKey, value);
+    }
+
+    async hgetall(key) {
+        return await this.client.hgetall(this.prefixKey + key);
+    }
+}
+
+module.exports = RedisStorage;
+```