Support R# annotations

[drewnoakes]

Create an external annotations XML file to enhance ReSharper's analysis capabilities for R# users, whilst continuing with no dependency upon JetBrains.Annotations.dll. Manually keeping such a file up to date would be highly error prone. Investigate creating a a Roslyn add-in that generates the external annotations at build time.

[drewnoakes]

Running...

devenv /ReSharper.Internal

...allows generating XML annotations. However attributes on type parameters do not seem to be included, nor are explicit [NotNull] annotations, in place of the implicit non-null feature used in this project.