[prowl] Initial contribution (#10967)

Signed-off-by: Ondrej Pecta <opecta@gmail.com>

bundles/org.openhab.binding.prowl/README.md

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+# Prowl Binding
+
+This is the binding for the [Prowl](https://www.prowlapp.com) iOS push service.  
+It has been written from scratch and therefore it is not based on the original 1.x Prowl binding.  
+It has no other purpose than sending push messages to iOS devices.
+
+## Binding Configuration
+
+The binding does not require any manual configuration on the binding level.
+
+## Thing Configuration
+
+This binding has only one thing called _Broker_. If you want to use this binding, just add a broker instance and configure the API key, which you can generate on the Prowl website.  
+You can also modify the _application_ property, which identifies the originator of these push messages.  
+If you want to have specific refresh time for the remaining free push messages channel, you can edit the _refresh_ property.
+Anyway beware - every check consumes one free push message you can send in an hour.
+
+## Channels
+
+The broker thing has only one channel keeping the number of free push messages, which can be sent.
+
+| channel    | type   | description                                            |
+|------------|--------|--------------------------------------------------------|
+| remaining  | Number | This channel provides the number of free push messages |
+
+## Example
+
+_*.things_
+
+```
+Thing prowl:broker:mybroker "Prowl Broker" [ apiKey="0000000000000000000000000000000000000000" ]
+```
+
+_*.rules_
+
+Once you have created the broker thing with a valid API key, you can use the Prowl service in your rules.  
+First you need to create an instance of the broker just before any call or on the top rules level. (replace the _mybroker_ with the right name of your instance).
+Then you can call method _pushNotification_, which requires two parameters - _event_ and _description_.  
+
+```
+val prowl = getActions("prowl","prowl:broker:mybroker")  
+prowl.pushNotification("Event", "This is the description of the event")
+```