Harmonize Net40Async csproj to other Polly projs
Harmonize Net40Async csproj to other Polly projs

Harmonise Polly.Net40Async.csproj to Polly.Net40.csproj.
Adds XML documentation file.
Harmonizes warnings-as-errors.
Ensures Polly.Net40Async..dll generated, will be semver version-number
stamped.

michael-wolfenden/Polly