[docs] add uninstall notes

docs/uninstall.md

+# How to uninstall Desktop LEAP VPN
+
+*applies to RiseupVPN, CalyxVPN, CodigoSurVPN etc...*
+
+## Windows
+
+1. Close the app.
+2. Open the folder where the application is installed (Usually `C:\Program Files\RiseupVPN\`).
+3. Click on `uninstall.exe`
+
+## OSX
+
+1. Close the app.
+2. Open the folder where the application is installed (Usually `/Applications/RiseupVPN/`).
+3. Click on `uninstall.app`
+
+
+## Linux 
+
+Check your package manager.