move drone config to repo root and add triggers

.drone.yml

@@ -0,0 +1,17 @@
+kind: pipeline
+name: test-on-amd64
+
+platform:
+  arch: amd64 
+
+steps:
+- name: clippy
+  image: rust:buster
+  commands:
+  - cargo clippy --verbose --all
+
+trigger:
+  event:
+    include:
+    - push
+    - pull_request