Add license to NuGet package

[danroth27]

Looks like the OmniSharp.Extensions.LanguageProtocol package doesn't have any license info.

[danroth27]

License also missing from the LanguageProtocol and JsonRpc packages.

[david-driscoll]

@danroth27 I can add that.

[danroth27]

@david-driscoll Thanks! What license do you plan to use for the packages? Will it be the same MIT license used for the source code?

[david-driscoll]

@danroth27 yeah, I'm going to do that right now.