Add ObservableCollections and ObservableCollections.R3

[shiena]

The NuGet package needs to respect a few constraints in order to be listed in the curated list:

• Add a link to the NuGet package: https://www.nuget.org/packages/ObservableCollections
• Add a link to the NuGet package: https://www.nuget.org/packages/ObservableCollections.R3
• It must have non-preview versions (e.g 1.0.0 but not 1.0.0-preview.1)
• It must provide .NETStandard2.0 assemblies as part of its package
• The lowest version added must be the lowest .NETStandard2.0 version available
• The package has been tested with the Unity editor
• The package has been tested with a Unity standalone player
  • if the package is not compatible with standalone player, please add a comment to a Known issues section to the top level readme.md
• All package dependencies with .NETStandard 2.0 target must be added to the PR (respecting the same rules above)
  • Note that if a future version of the package adds a new dependency, this dependency will have to be added manually as well

Note: The server will be updated only when a new version tag is pushed on the main branch, not necessarily after merging this pull-request.