Add `.gitmodules` file to fix errors reported by SourceTree when checking out releases. Somehow these definitions came across in a recent objectstore merge but the relative paths were wrong for our nested structure.
Add `.gitmodules` file to fix errors reported by SourceTree when checking out releases. Somehow these definitions came across in a recent objectstore merge but the relative paths were wrong for our nested structure.

realm/realm-dotnet,realm/realm-dotnet,Shaddix/realm-dotnet,realm/realm-dotnet,Shaddix/realm-dotnet,Shaddix/realm-dotnet,Shaddix/realm-dotnet