Updated to netcoreapp3.1

* added basic sample

* updated docs

samples/OcelotBasic/ocelot.json

@@ -0,0 +1,21 @@
+{
+  "ReRoutes": [
+    {
+      "DownstreamPathTemplate": "/todos/{id}",
+      "DownstreamScheme": "https",
+      "DownstreamHostAndPorts": [
+        {
+          "Host": "jsonplaceholder.typicode.com",
+          "Port": 443
+        }
+      ],
+      "UpstreamPathTemplate": "/posts/{id}",
+      "UpstreamHttpMethod": [
+        "Get"
+      ]
+    }
+  ],
+  "GlobalConfiguration": {
+    "BaseUrl": "https://localhost:5000"
+  }
+}