Add CHANGELOG.md

CHANGELOG.md

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+Next Version
+------------
+
+BREAKING CHANGES:
+- The OpenLayers version was updated to v6.5.0. To use this version run `npm i @geoext/openlayers-legacy` and include the
+`ol.js` and `ol.css` files from there.
+- Due to the OpenLayers update the OverviewMap no longer can use the same layers as the main map and always has to be
+provided own layers.
+- One of the bigger changes in OpenLayers is the removal of `opt_this` arguments. If you do not call `un` on the method
+you can simply use `.bind` on the call. It you want to unbind the listener, either store a bound version of the listener
+or store the listenerKey returned by `on` and usw `ol.Observable.unByKey`.
+- Please refer to https://github.com/openlayers/openlayers/releases/tag/v5.0.0 and
+https://github.com/openlayers/openlayers/releases/tag/v6.0.0 for overviews over the OpenLayers changes.