Add README to .nuspec

No code change, just adding the README to the NuGet package.

Multi-targeting and General Cleanup

This release includes possible breaking changes in the way that null/missing elements/properties are handled. This library will throw more often than before in the case of improperly formatted STL data.

1.3.1

https://www.nuget.org/packages/QuantumConcepts.Formats.STL/1.3.1

Version 1.1.0.0

Includes enhancements and bug fixes as indicated in the 1.1.0.0 milestone.

Version 1.0.0.0 - Initial Release

This is the initial release which includes all basic features required to read/write STL files.