Adding release notes for Android 4.3.0

[mmattel]

Adding release notes for Android 4.3.0

Note, there was a thread adding important stuff as text description.
We could add that easily here or need to file another PR post merging.

We could also remove not that relevant stuff like:

• Add new prefixes in commit messages of 3rd party contributors:
  https://github.com/owncloud/android/issues/4346[#4346]
• Kotlinize PreviewTextFragment:
  https://github.com/owncloud/android/issues/4356[#4356]
• Unit tests for repository classes - Part 1:
  https://github.com/owncloud/android/issues/4232[#4232]
• ...

[phil-davis]

Text looks OK

[jesmrec]

I'd structure the 4.3.0 release notes

Important items:

• New Dialog for Manage Accounts
• Support for URL shortcut files
• New setting to remove automatically downloaded files
• New setting to remove manually local storage
• Automatic discovery
• Password generator

Other improvements:

• Feedback section changed
• Device's clear button re-enabled
• Filtering in spaces list
• Performance in av. offline
• Warning for http connections
• Bugfixing, accessibility and tiny UI improvements

[mmattel]

I have removed the items listed on top and added the suggestion from @jesmrec.
Looks good when rendered locally, imho ready to approve and merge.

[jesmrec]

If i understand correctly, General is like the header to read at a glance and the others Issues, Changes and Enhancements are the detailed information. Everything in General is in the other sections, but not everything in the other sections is in General, right @mmattel ?