📝 Add documentation around Redis auth

docs/redis_authentication.md

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+# Redis Authentication
+
+To avoid any unauthorized access to the Redis database resulting in an unintended behavior, arising from the usage of Redis' passwordless `default` user.
+
+With Lisk Service [v0.7.2](https://github.com/LiskHQ/lisk-service/tree/v0.7.2), similar to our MySQL setup, we now enforce our custom authentication on Redis, by default, with the following credentials:
+
+> **Username**: `lisk` <br>
+> **Password**: `password`
+
+To programmatically connect to Redis, please specify the authentication details in the connection string in the following format:
+
+```
+redis://<username>:<password>@<redis_server_ip>/<database_number>
+```
+> **Example**: redis://lisk:password@127.0.0.1/0
+
+## Setting up new authenticated Redis instances
+
+- Starting a dockerized Lisk Service instance from scratch should automatically take care of setting up proper authentication on Redis.
+
+- When self-hosting Redis instances (PM2 users) please look into our following Redis config files:
+  - [redis.persistent.conf](../docker/redis.persistent.conf)
+  - [redis.volatile.conf](../docker/redis.volatile.conf)
+
+When using a custom Redis config, please consider adding the following ACL configurations:
+
+```
+# ACL rule for a lisk user
+
+requirepass password
+user lisk on allkeys allchannels allcommands >password
+user default off
+```
+
+## Setting up auth on existing passwordless Redis instances
+
+To set up authentication on your current running Redis instance, execute the following commands after you login to the Redis CLI:
+
+```
+CONFIG SET requirepass password
+ACL SETUSER lisk on allkeys allchannels allcommands >password
+ACL SETUSER default off
+```
+
+Verify that the changes are applied successfully with the following commands:
+
+**Executing a command without authentication**:
+```
+/data # redis-cli info cpu
+NOAUTH Authentication required.
+```
+
+**Executing a command with the authentication details**:
+```
+/data # redis-cli --user lisk --pass password info cpu
+Warning: Using a password with '-a' or '-u' option on the command line interface may not be safe.
+# CPU
+used_cpu_sys:1.359498
+used_cpu_user:1.334618
+used_cpu_sys_children:0.001210
+used_cpu_user_children:0.000709
+used_cpu_sys_main_thread:1.358968
+used_cpu_user_main_thread:1.334302
+```
+
+> **NOTE**: To ensure smooth operation of Lisk Service post the current change, kindly restart the Redis server after authentication is set up.
+
+## Using Redis auth-free
+
+In case, you have an existing passwordless running instance of Redis and want to continue using it, please override all the Redis connection strings via the necessary environment variables either in the `.env` (Docker setup) or the `ecosystem.config.json` (PM2 setup) config files.
+
+To easily find the necessary environment variables, open the relevant config file, and replace all the instances of `redis://lisk:password@` with `redis://`. Please ensure that the update environment variables are enabled.
+
+> **NOTE**: We _**highly recommend**_ using authentication on Redis.