Configure Dependabot for dotnet-sdk updates (#524)

Author: torosent

.github/dependabot.yml

@@ -0,0 +1,21 @@
+version: 2
+updates:
+  - package-ecosystem: "nuget"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "tuesday"
+    ignore:
+      - dependency-name: "*"
+        update-types: 
+          - "version-update:semver-major"
+
+  - package-ecosystem: "dotnet-sdk"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "tuesday"
+    ignore:
+      - dependency-name: "*"
+        update-types: 
+          - "version-update:semver-major"