Update README.md

[VincentMeilinger]

Topic and Scope

This PR adds another cookbook client to the list of mobile clients in the Readme.

Concerns/issues

This should not cause any issues.

Formal requirements

There are some formal requirements that should be satisfied. Please mark those by checking the corresponding box.

• I did check that the app can still be opened and does not throw any browser logs
• I created tests for newly added PHP code (check this if no PHP changes were made)
• I updated the OpenAPI specs and added an entry to the API changelog (check if API was not modified)
• I notified the matrix channel if I introduced an API change
  (There is no API change, and this should not affect whether the app opens)