Add note about Global Privacy Control (GPC)

[Global Privacy Control](https://globalprivacycontrol.org/) automates browser-level opt-outs under privacy legislation such as CCPA/CPRA and can be enabled in [Firefox with a setting](https://blog.mozilla.org/netpolicy/2021/10/28/implementing-global-privacy-control/) or Chrome with an extension such as [Privacy Badger](https://privacybadger.org/). It’s on by default in Brave.