Separate section for Joplin configuration.

Joplin.md

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+#### Auth Token
+You can enter your auth token on your computer by saving an empty token. Then quit KOReader, edit the `exporter.joplin_token` field in `settings.reader.‍lua` after creating a backup, and restart KOReader once you're done.
+
+#### Port Forwarding
+To export to Joplin, you must forward the IP and port used by this plugin to the `localhost:port` on which Joplin is listening. This can be done with `socat` or a similar program. For example:
+
+For Windows:
+
+```cmd
+netsh interface portproxy add v4tov4 listenaddress=0.0.0.0 listenport=41185 connectaddress=localhost connectport=41184
+```
+
+For Linux:
+
+```bash
+socat tcp-listen:41185,reuseaddr,fork tcp:localhost:41184
+```