Add Dependabot config file (#939)

.github/dependabot.yaml

@@ -0,0 +1,40 @@
+# Summary: Dependabot config file for OpenFermion.
+#
+# By default, Dependabot labels all pull requests with label 'dependencies'.
+# We use `dependency bug`, so have to configure Dependabot appropriately.
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+version: 2
+updates:
+  - package-ecosystem: "docker"
+    # The "docker" ecosystem directive makes Dependabot look for a Dockerfile
+    # in the specified directory.
+    directories:
+      - "/docker/"
+      - "/dev_tools/requirements/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "dependency bug"
+      - "docker"
+
+  - package-ecosystem: "github-actions"
+    # The "github-actions" code explicitly looks in /.github/workflows if the
+    # value "/" is given for the directory attribute. Yes, that's confusing.
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    labels:
+      - "dependency bug"
+      - "github workflows"
+
+  - package-ecosystem: "pip"
+    # OpenFermion has requirements.txt files in multiple places.
+    directories:
+      - "/dev_tools/requirements/"
+      - "/docs/tutorials/"
+    schedule:
+      interval: "weekly"
+    versioning-strategy: "increase-if-necessary"
+    labels:
+      - "dependency bug"