README for 3.2.0

[mfl28]

Brings the README up to date for the 3.2.0 release.

• Features: automatic saving (Autosave unexported annotations and offer to restore them #226). Annotations that aren't exported yet are saved a few seconds after each change and offered for restore after a crash or a forced quit. The item links to the wiki section "Automatic saving".
• Import and export: JSON now keeps masks, so it has its own line ("rectangles, polygons and masks, including nesting") instead of sharing one with Pascal VOC.

The download links are unchanged: the Version Bump workflow updates them to 3.2.0 after the release.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (6e25751) to head (a758352).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage     90.29%   90.29%           
  Complexity     2392     2392           
=========================================
  Files           140      140           
  Lines          8988     8988           
  Branches        863      863           
=========================================
  Hits           8116     8116           
  Misses          452      452           
  Partials        420      420           

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