docs: add instructions for Google accounts

This makes it easier for people to try out koushin.

docs/google.md

@@ -0,0 +1,20 @@
+# Running koushin with a Google account
+
+## Create an application password
+
+First, you'll need to obtain an application-specific password for koushin from
+the [app passwords] page on your Google account.
+
+## Run koushin
+
+Start koushin with these upstream URLs:
+
+    koushin imaps://imap.gmail.com smtps://smtp.gmail.com \
+        carddavs://www.googleapis.com/carddav/v1/principals/YOUREMAIL/
+
+Replace `YOUREMAIL` with your Google account's e-mail address.
+
+Once koushin is started, you can login with your e-mail address and the app
+password.
+
+[app passwords]: https://security.google.com/settings/security/apppasswords

go.mod

@@ -25,3 +25,5 @@ require (
 	golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5 // indirect
 	layeh.com/gopher-luar v1.0.7
 )
+
+replace github.com/emersion/go-webdav => ../go-webdav