Adding the OkSo drawing app to the list

Adding the [okso.app](https://okso.app) drawing app that has the possibility to organize sketches in the nested "drawing inside the drawing" structure

readme.md

@@ -1322,6 +1322,7 @@
 | [devices.css](http://marvelapp.github.io/devices.css/) | Pure CSS phones and tablets by Marvel App. |
 | [Troopl](https://troopl.com) | Build and publish a free portfolio in minutes. |
 | [Apifox](https://apifox.cn) | Apifox = Postman + Swagger + Mock + JMeter |
+| [okso.app](https://okso.app) | The drawing app with a nested "drawing-inside-the-drawing" structure |
 
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