Document in known issues the workaround for wrong keyboard layout being selected

See #7912 (closed). Then, the commit(s) that add this text to known issues will need to be reverted in feature/jessie.

So, the workaround is: click on EN in the Top navigation bar, your chosen keyboard will be enabled.

Feature Branch: bitingbird:doc/9336-keyboard_bug

Original created by @intrigeri on 9336 (Redmine)

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