added infos on how to add a 1.x add-on to the distro (openhab#190)

Signed-off-by: Kai Kreuzer <[email protected]>

docs/sources/features/compatibilitylayer.md

@@ -24,7 +24,15 @@ Here is what you need to do:
  - If it compiles, the first major step is already done. If not, try to figure out why there are compilation problems and if you cannot solve them, ask on the mailing list for help. 
  - After adding some configuration, start up the runtime through the launch configuration (make sure your bundle is activated and started by default) from within the IDE.
  - Go and test and report your findings by creating issues or pull requests for the [add-on in openHAB 1](https://github.com/openhab/openhab/issues).
- - If all is well and the binding works, add it to the official distribution by creating a PR similar to [this one](https://github.com/openhab/openhab/pull/3720/files). With will automatically make the add-on a part of the distro with the next build.
+
+## How to add a successfully tested 1.x Add-on to the distribution
+
+1. The first step is to create a "Karaf feature" for it, in which required dependencies (i.e. to io.transport bundles) can also be declared. Such a feature can also include the required configuration, therefore you should create a file `<youraddon>.cfg` in `features/openhab-addons-external/src/main/resources/conf` with the same content as what is within `openhab.cfg`, but without the `<yourbinding>:` prefix on the lines of the parameters. 
+This config file then needs to be added to `features/openhab-addons-external/pom.xml` so that the build is aware of it.
+The feature itself is then added to `features/openhab-addons/src/main/feature/feature.xml`, referencing the bundle, the config file and its dependencies (if any). The result should look [similar to this](https://github.com/openhab/openhab/pull/3988/files).
+This will automatically make the add-on a part of the distro with the next build.
+1. Note that with defining a Karaf feature, bindings are available for installation through the Paper UI in the "Extensions" menu, but they are not listed under "Configuration->Bindings" (although it is fully operational after installation). In order to have bindings listed there as well, you need to add some meta-information to the binding bundle. This information should be put into `ESH-INF/binding/binding.xml` and its content is [described here](https://www.eclipse.org/smarthome/documentation/development/bindings/xml-reference.html#xml-structure-for-binding-definitions). Do not forget to add `ESH-INF` to your `build.properties`, so that it is packaged in the bundle. See a [real life example of such meta-data here](https://github.com/openhab/openhab/pull/4271/files).
+
 
 ## Future Plans