ci: add Drone CI

This is my first experiment to make sure I can actually use this in my
CI.

.drone.yml

@@ -0,0 +1,27 @@
+---
+kind: pipeline
+type: exec
+name: Matrix Notifier check
+
+steps:
+- name: Pre-commit checks
+  commands:
+  - nix flake check
+
+- name: Notifiy
+  commands:
+  - nix run .
+  environment:
+    ADDRESS:
+      from_secret: matrix_homeserver
+    ROOM:
+      from_secret: matrix_roomid
+    USER:
+      from_secret: matrix_username
+    PASS:
+      from_secret: matrix_password
+  when:
+    status:
+      - failure
+      - success
+...