Consider adding R# annotations

[GoogleCodeExporter]

ReSharper is very widely used, and has some annotations which we could apply 
within Noda Time to indicate nullity, purity etc:

http://www.jetbrains.com/resharper/webhelp/Code_Analysis__Annotations_in_Source_
Code.html

We definitely wouldn't want to add a binary dependency, and we may well want to 
move the annotations to another namespace, but that would still be useful:

- We'd get the benefits ourselves when developing Noda Time
- Other developers could configure R# to notice the Noda Time copy of the 
annotations too, at which point it would be able to help them use Noda Time 
more effectively.

It may be worth trying to find other examples of open source projects doing 
this, to see how they've found it.

Original issue reported on code.google.com by jonathan.skeet on 4 Apr 2013 at 8:31