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Merge pull request #12650 from savonarola/0305-refine-retainer-readme 

chore(retainer): actualize README
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EMQX Retainer
==============
# Retainer
The retainer plugin is responsible for storing retained MQTT messages.
The `emqx_retainer` application is responsible for storing retained MQTT messages.
Configuration
-------------
The retained messages are messages associated with a topic that are stored on the broker and delivered to any new subscribers to that topic.
etc/emqx_retainer.conf:
More information about retained messages can be found in the following resources
* [MQTT specification 3.3.1.3](http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html).
* Retained message concept in [EMQX documentation](https://www.emqx.io/docs/en/v5.0/messaging/mqtt-concepts.html#retained-message).
* Instructions for publishing retained messages in [EMQX documentation](https://www.emqx.io/docs/en/v5.0/messaging/mqtt-retained-message.html#publish-retained-message-with-mqttx-client).
* [The Beginner's Guide to MQTT Retained Messages](https://www.emqx.com/en/blog/mqtt5-features-retain-message).
## Usage
The `emqx_retainer` application is enabled by default. To turn it off, add the following configuration to the `emqx.conf` file:
```
## Where to store the retained messages.
## Notice that all nodes in a cluster are to have the same storage_type.
##
## Value: ram | disc
## - ram: memory only
## - disc: both memory and disc
##
## Default: ram
retainer.storage_type = ram
## Maximum number of retained messages allowed.
##
## Value: Number >= 0
retainer.max_retained_messages = 1000000
## Maximum payload size of a retained message.
##
## Value: Bytes
retainer.max_payload_size = 64KB
## Expiration interval of the retained messages. Never expire if the value is 0.
##
## Value: Duration
## - h: hour
## - m: minute
## - s: second
##
## Examples:
## - 2h: 2 hours
## - 30m: 30 minutes
## - 20s: 20 seconds
##
## Default: 0
retainer.expiry_interval = 0
retainer {
enable = false
}
```
License
-------
For other options, see the [configuration](https://www.emqx.io/docs/en/v5.2/configuration/configuration-manual.html#retainer) documentation.
Apache License Version 2.0
## Contributing
Author
------
Please see our [contributing.md](../../CONTRIBUTING.md).
EMQX Team
## License
See [LICENSE](../../APL.txt)