Create global.json

Tells the .NET Core compiler where the source directory is, so when commands like `dotnet restore` are run, the solution location can be found. This is to help make the build work in AppVeyor.

Author: joelhulen

global.json

@@ -0,0 +1,5 @@
+{
+    "projects": [
+        "src"
+    ]
+}