Add notes on bump

CONTRIBUTING.md

@@ -54,4 +54,32 @@ $ docker-compose stop
 
 ## Production
 
+Bump the tag in [.drone.yml](./.drone.yml) and [docker-compose.production.yml](./docker-compose.production.yml).
+
+Here's an example git diff. Please update the change log too.
+
+```diff
++++ b/.drone.yml
+@@ -10,7 +10,7 @@ steps:
+       password:
+         from_secret: docker_reg_passwd
+       repo: decentral1se/autonomic.zone
+-      tags: v2.0.0
++      tags: v3.0.0
+
+   - name: deployment
+     image: decentral1se/drone-stack:19.03.8-override-compose
+
++++ b/docker-compose.production.yml
+@@ -3,7 +3,7 @@ version: "3.7"
+
+ services:
+   jekyll:
+-    image: decentral1se/autonomic.zone:v2.0.0
++    image: decentral1se/autonomic.zone:v3.0.0
+     command: jekyll serve --host 0.0.0.0 --trace
+     environment:
+       JEKYLL_ENV: production
+```
+
 Our [drone.autonomic.zone](https://drone.autonomic.zone/autonomic-cooperative/autonomic.zone/) configuration automatically deploys.