Add Google Play link for AvoCook client

[Logarex]

Topic and Scope

This PR adds the Google Play Store badge and reference link for the AvoCook mobile client to the list of mobile applications.

Concerns/issues

None. This is purely a documentation update in the README.

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

[github-actions]

Test Results

   12 files    592 suites   2m 36s ⏱️
  589 tests   581 ✅  8 💤 0 ❌
2 356 runs  2 323 ✅ 33 💤 0 ❌

Results for commit c763bac.