build: add initial Mergify configuration

This allows contributors to the repository to automatically merge PRs
when it has two positive reviews.

Only rebased merges should be done, no squashing od commits or inclusion
of merge commits.

Signed-off-by: Niels de Vos <[email protected]>

.github/mergify.yml

@@ -0,0 +1,26 @@
+---
+defaults:
+  actions:
+    merge:
+      method: rebase
+      rebase_fallback: merge
+      strict: smart
+      strict_method: rebase
+
+pull_request_rules:
+  - name: remove outdated approvals
+    conditions:
+      - base~=^(main)|(release-.+)$
+    actions:
+      dismiss_reviews:
+        approved: true
+        changes_requested: false
+  - name: automatic merge
+    conditions:
+      - base~=^(main)|(release-.+)$
+      - "#approved-reviews-by>=2"
+      - "#changes-requested-reviews-by=0"
+    actions:
+      merge: {}
+      dismiss_reviews: {}
+      delete_head_branch: {}