setup "main" branch and update Readme

[TimothyMothra]

Fix Issue #2466

Changes

• Update branches description for "main"
• Update nuget badge for both official stable and official pre-release
• Update all build definitions to use "main" branch

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

Notes for reviewers:

• We support comment build triggers
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