Add MIT license

Open-source under MIT - free for any use including commercial, modify
+ redistribute freely, must preserve the copyright + license notice in
copies, and the standard AS-IS no-warranty / no-liability clauses.

Also sets PackageLicenseExpression=MIT on the project metadata so any
NuGet packages produced from this codebase get the right SPDX tag.

Directory.Build.props

@@ -9,6 +9,7 @@
     <PackageProjectUrl>https://jpaul.me</PackageProjectUrl>
     <RepositoryUrl>https://github.com/recklessop/webhook-server</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
   </PropertyGroup>
 
 </Project>