Add license to nuget package (#1657)

This adds a license to the nuget package.
The most visible places are in the nuget package manager and on https://www.nuget.org/packages/PuppeteerSharp.

Author: jnyrup

lib/PuppeteerSharp/PuppeteerSharp.csproj

@@ -6,6 +6,7 @@
     <Authors>Darío Kondratiuk</Authors>
     <Owners>Darío Kondratiuk</Owners>
     <PackageProjectUrl>https://github.com/hardkoded/puppeteer-sharp</PackageProjectUrl>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <Summary>Headless Browser .NET API</Summary>
     <PackageTags>headless,chrome,puppeteer</PackageTags>
     <Title>PuppeteerSharp</Title>