Add FAQ, discuss getter topic (#87)

* Add FAQ, discuss getter topic

* Update README.md

README.md

@@ -30,6 +30,8 @@ Implementations of the Homie convention can be found on [this page](implementati
     * [Property attributes](#property-attributes)
   * [Arrays](#arrays)
   * [Broadcast channel](#broadcast-channel)
+* [FAQ and Rationale](#faq)
+
 
 ## Motivation
 
@@ -584,3 +586,23 @@ homie/$broadcast/alert ← "Intruder detected"
 ```
 
 Any other topic is not part of the Homie convention.
+
+----
+----
+
+## FAQ
+
+In this section frequently asked questions will be answered.
+This includes design decisions and drawn compromises in the specifics of the Homie convention.
+
+### How do I query/request a property?
+
+You don't.
+The MQTT protocol does not implement the request-reply but rather the publish-subscribe messaging pattern.
+The Homie convention follows the publish-subscribe principle by publishing data as retained messages on a regular basis.
+You might want to rethink the design of your application - in most scenarios a regularly updated information is sufficient.
+
+*Workaround:* You are free to implement your own ideas on top of the basic structure of the Homie convention.
+You could either implement a `get` getter topic and its logic to trigger a value update, or you may exploit the concept of Homie properties and define a settable property to trigger a value update.
+
+A discussion on the matter can be found in issue [#79](https://github.com/homieiot/convention/issues/79).