.NET Core Support

I notice there is Docker support and that it should, in theory, allow .NET Core support (mentioned in #845).

I'm wondering if this has been tested and/or should work out of the box or if there is more work to be done. Because the docs don't mention .NET Core.

If there is more work to be done, I'd be glad to help. I've tried running the aws-csharp example (with a slightly modified serverless-offline plugin to allow `dotnetcore` runtime), but I get an error:

```
AWSLambda.Internal.Bootstrap.LambdaValidationException: Could not find the specified handler assembly with the file name 'CsharpHandlers, Culture=neutral, PublicKey
Token=null'. The assembly should be located in the root of your uploaded .zip file.
```

And indeed, there is no zip file in my `.serverless` folder. Though I have the same issue when I run `serverless invoke local -f hello`.

So my question is this: do you want me to investigate this so we can confirm serverless-offline supports .NET Core? Or do you already know it does (and then the issue I'm having is something I should fix myself)?

Start by reproducing the failure with the aws-csharp example, serverless-offline configured for the dotnetcore runtime, and `serverless invoke local -f hello`. Inspect the generated `.serverless` contents and the handler assembly packaging path. Done means confirming whether .NET Core works, identifying the missing work if it does not, and updating the documentation if support is confirmed.

Written by the indexing model from the issue text.