Add Theengs links (#1249)

docs/participate/support.md

@@ -3,6 +3,7 @@
 If you like the project and/or used it please consider supporting it! It can be done in different ways:
 * Helping other users in the [community](https://community.openmqttgateway.com)
 * [Contribute](development) to the [code](https://github.com/1technophile/OpenMQTTGateway) or the [documentation](https://docs.openmqttgateway.com)
+* Buy the [Theengs mobile application](https://app.theengs.io)
 * Buy devices, boards or parts from the [compatible web site](https://compatible.openmqttgateway.com), the devices and parts linked use affiliated links.
 * Donate or sponsor the project [developers](https://github.com/1technophile/OpenMQTTGateway/graphs/contributors)
 * Make a video or a blog article about what you have done with [OpenMQTTGateway](https://docs.openmqttgateway.com) and share it.