How can I create an AddIn to ArcGIS Pro and where can I find it from ArcGIS Pro application?
ArcGIS Pro currently does not support Python add-ins. There were no plans for a Python Add-in support in ArcGIS Pro. ArcGIS Pro has arcpy site package shipped though (you can author Python script tools).
Your only option is ArcGIS Pro .NET SDK which is going to be released soon (it is in beta now).
UPDATE: from Esri Forum
ArcGIS Pro will feature a new .Net API using coarse grained objects which we currently (and internally) calling ProObjects. Add-Ins based on the ArcObjects APIs will not work in Pro. Think of the architecture difference of ArcMap and Pro being somewhat related when you make a comparison between ArcGIS Engine and the ArcGIS Runtime.
Getting an add-in into Pro is similar to getting an add-in into ArcMap. The backstage in Pro is featuring an Add-In Manager that works in a similar way to the classic desktop applications.