fix: #541 package nuspec warnings

[miquelbeltran]

This PR fixes the nuspec warnings.

Output of nuget tool:

mono ~/bin/nuget.exe pack raygun4js.nuspec        
Attempting to build package from 'raygun4js.nuspec'.
Successfully created package '/[redacted]/raygun4js/raygun4js.3.1.4.nupkg'.

Closes #541

[Olwiba]

LGTM, I don't know how relative files work in .nuspec files, but I confirmed there is a LICENSE file in the project root and this is pointing at it correctly. Guessing this bundles this in the package?

[miquelbeltran]

Yes, the file tag takes care of bundling it, I also added the README.md to it as it was missing.