Add cross-reference for app and TAN hotline operating hours

[MikeMcC399]

This PR adds a cross-reference from this hotline repository to the external website https://www.coronawarn.app/ regarding the operating hours of the hotlines. The following sentence is added to the README file:

"The operating hours of the app and TAN hotlines are listed in the Support section of the Corona-Warn-App website."

So that the operating hours do not have to be maintained in multiple places, the above does not list the hours literally, it only provides a cross-reference to https://www.coronawarn.app/en/faq/#support.

Current / planned changes to operating hours

• The current operating hours of the App hotline are Monday to Saturday from 7 a.m. to 10 p.m.
• According to issue https://github.com/corona-warn-app/cwa-website/issues/2294 the TAN hotline changed its operating hours on Jan 16, 2022 to Monday to Sunday from 7 a.m. to 8 p.m.

[MikeMcC399]

@dsarkar Please consider this PR. It coincides with the change of operating hours for the TAN hotline.

[dsarkar]

@Ein-Tim @MikeMcC399 Thanks.