Update README.md

Add notice about password-manager-service to README

README.md

@@ -26,12 +26,25 @@ It works on Ubuntu, Fedora, Debian, and other major Linux distributions.</p>
 
 ## Install
 
-    sudo snap install protonmail-bridge
+```shell
+sudo snap install protonmail-bridge
+```
 
 [![Get it from the Snap Store](https://snapcraft.io/static/images/badges/en/snap-store-white.svg)](https://snapcraft.io/protonmail-bridge)
 
 ([Don't have snapd installed?](https://snapcraft.io/docs/core/install))
 
+## Snap configuration
+
+You need to have a keychain in order to run the Proton Mail Bridge. To allow
+access to the keychain, you have to manually connect the
+[`password-manager-service`](https://snapcraft.io/docs/password-manager-service-interface)
+plug.
+
+```shell
+sudo snap connect protonmail-bridge:password-manager-service
+```
+
 ## Remaining tasks
 
 Snapcrafters ([join us](https://forum.snapcraft.io/t/snapcrafters-reboot/24625)) are working to land snap install documentation and the [snapcraft.yaml](https://github.com/snapcrafters/fork-and-rename-me/blob/master/snap/snapcraft.yaml) upstream so [Project] can authoritatively publish future releases.