Add upgrade instructions

Fixes baskerville#159.

doc/GUIDE.md

@@ -7,3 +7,7 @@ Pick one of the [one-click install packages](https://www.mobileread.com/forums/s
 The settings are saved in and read from `Settings.toml`. You can edit this file when *Plato* isn't running or is in shared mode. You can enter the shared mode by connecting your device to a computer.
 
 The default ePUB stylesheet, `css/epub.css`, can be overriden via `css/epub-user.css`.
+
+## Upgrade
+
+Install the corresponding one-click package on top of the previous one. Check out the [release notes](https://github.com/baskerville/plato/releases) before upgrading: manual intervention might be required.