Please document migrating to new authentication scheme from slack API token

[gabor-braun]

In Pidgin I was unable to change the settings (username, host) until January 4, when there was a Slack outage. Then selecting to change settings in the error popup, I could alter the username and host as README recommends.

Once Slack was available, I could connect with the new settings, however all the logs became empty, when selecting a contact and asking for the log in the popup menu. (By the way, the logs are still in my filesystem in ~/purple/logs/slack/EMAIL. The slack account name as shown in Pidgin has changed from EMAIL to EMAIL%SLACK-HOST.)

Given this experience I would like to see instruction for migrating to the new authentication scheme which preserves all other settings, including logs of previous conversation. Downloading logs again from Slack shouldn't be the solution, as I have another account yet to migrate with more history than Slack allows to download.