feat: add mergify configuration file (#36)

Add a
[mergify.yml](https://github.com/TestableIO/System.IO.Abstractions/blob/main/.github/mergify.yml)
file.

Author: vbreuss

.github/mergify.yml

@@ -0,0 +1,16 @@
+pull_request_rules:
+  - name: Automatic merge on approval
+    conditions:
+      - "#approved-reviews-by>=1"
+    actions:
+      merge:
+        method: squash
+        commit_message_template: |-
+          {{ title }} (#{{ number }})
+          {{ body }}
+  - name: Automatic merge for Renovate pull requests
+    conditions:
+      - author=renovate[bot]
+    actions:
+      merge:
+        method: squash