[infrastructure] add upgrade instructions to readme (#265)

Closes #261

Signed-off-by: Jan N. Klug <github@klug.nrw>

README.md

@@ -15,6 +15,10 @@ The documentation can be found [here](https://github.com/smarthomej/addons/tree/
 For openHAB 3.2.0 and later it is recommended to add `https://download.smarthomej.org/addons.json` as JSON 3rd Party Addon Service in the settings.
 Afterwards all addons can be installed from the UI.
 
+If you installed SmartHome/J bindings from Community Marketplace (NOT the "JSON 3rd Party Addon Service" mentioned above!), you need to manually remove the installed addons.
+Go to your `userdata` folder (e.g. `/var/lib/openhab` on Debian systems) and remove the marketplace directory.
+*Attention:* This will also uninstall all other bindings installed from the marketplace, make sure you install everything you need afterwards.
+
 ### Compatibility
 
 Due to a breaking changes in openHAB, older and newer versions of bundles are not compatible in every combination.
@@ -27,6 +31,28 @@ Please check the table to see which versions of SmartHome/J are compatible with
 | SmartHome/J 3.2.x (snapshots, releases) | no | yes | yes| yes |
 | compatible RepoManager | [latest 3.1.x](https://download.smarthomej.org/repomanager-latest) | [latest 3.2.x](https://download.smarthomej.org/repomanager-latest-3.2.x) | JSON AddonService | JSON AddonService | 
 
+### Upgrading your openHAB system from 3.0.x/3.1.x to 3.2.x
+
+While openHAB is running (this is important, otherwise caching might be an issue!):
+
+- uninstall all SmartHome/J Bindings
+- delete the RepoManager jar from your addons folder (if present)
+- delete the .kar with all addons from the addons folder (if present)
+
+Stop openHAB, perform the upgrade and follow the installation instructions for openHAB 3.2.x above.
+
+### Upgrading SmartHome/J bindings installed from the JSON 3rd Party Add-on service
+
+The newest version (and only that !) is available for installation in the UIs Add-ons section.
+You'll always install the newest version, but installed addons keep their version.
+
+Unfortunately there is no automatic or half-automatic update process.
+If you want to upgrade after a new version is released, you have to manually uninstall and re-install the binding.
+Your configurations (binding-configurations and thing configurations) are safe and will be picked up by the new version.
+Things will automatically update their type/definition where necessary.
+
+*Attention:* Even though we try to reduce breaking changes to an absolute minimum, please always look at the release notes prior to updating.
+
 ## Development
 
 SmartHome/J add-ons are [Java](https://en.wikipedia.org/wiki/Java_(programming_language)) `.jar` and [Apache Karaf](https://karaf.apache.org) `.kar` files.