Add changelog

[mesqueeb]

I was wondering if it's safe to update to 1.2.0 but I can't find a changelog : )

I think a changelog would be a great addition to this plugin! < 3

[ryaa]

This will be very useful to list all the changes for each release (especially breaking changes!). Here is a good example - see https://github.com/dpa99c/cordova-diagnostic-plugin/blob/master/CHANGELOG.md

[jcesarmobile]

The project now has a CHANGELOG file, so closing https://github.com/capacitor-community/camera-preview/blob/master/CHANGELOG.md