Make users aware that they can use transripted umlauts

[janschreiber]

As per #16, OpenThesaurus now supports transcripted umlauts and 'ss' for 'ß'. Users should be made aware of this new functionality in some way. I suggest an unobtrusive balloon tip that is triggered when the input field gets focus. The balloon could say something like 'Tip: Type ae for ä and ss for ß'. Also, it should have a close button and ideally stay closed for the rest of the session after the user closed it.

[danielnaber]

This should only be displayed for users that find it useful. Most users are from German-speaking countries and won't need this. Unfortunately it's not possible to detect what kind of keyboard a user has from Javascript, we could only detect the browser language and display this for browsers not configured for German.