Add Android client

[lneugebauer]

Topic and Scope

I added my Android client to the README.

Concerns/issues

No concerns.

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

     27 files  1 233 suites   7m 8s ⏱️
   498 tests    498 ✔️ 0 💤 0 ❌
4 482 runs  4 481 ✔️ 1 💤 0 ❌

Results for commit f32a1d0.

♻️ This comment has been updated with latest results.