Add Features section to README - Fixes #30

[viralynn]

What changed?

Added a short "Features" section to README.md, right after the introduction. It has two lists, Students and Mentors, written in simple English (14 lines of text). No other README section was changed.

Note: The intro already says there is no login, and the new section repeats it because the issue asks for that mention. Happy to remove one of the two if you prefer; I recommend removing the intro "no login" mention.

Which issue does this address?

Fixes #30

How was it tested?

This change only edits documentation, so I did not run composer test, flutter analyze or flutter test.

I checked every item by hand against the running app (backend with composer start, Flutter app with flutter run -d web-server --web-port 8081, Flutter 3.47.6 on Ubuntu 26.04.1 under WSL2), using the web browser:

• Student side: search box, Open/Closed/All filter, Newest/Most waiting sort, join form (name, email, topic, question), position and "students ahead" screen, and the 10-second automatic refresh.
• Mentor side: starting a session, selecting an open session (closed sessions do not appear in the dropdown), statistics, Take next student, Mark solved and Skip.
• Started a new session from the mentor dashboard; the app showed "Session created. It is selected below." and listed it in the Open session dropdown. It also appeared in the student session list after a manual refresh.
• Confirmation dialogs appear for Skip and for Close session. Mark solved has none.
• Checked git diff --stat: 1 file changed, 20 insertions, 0 deletions.