Fix drone CLI install link

Fixes go-gitea#4070

Author: stevegt

CONTRIBUTING.md

@@ -69,7 +69,7 @@ and keep the compatibility on upgrade. To make sure you are
 running the test suite exactly like we do, you should install
 the CLI for [Drone CI](https://github.com/drone/drone), as
 we are using the server for continous testing, following [these
-instructions](http://readme.drone.io/usage/getting-started-cli). After that,
+instructions](http://docs.drone.io/cli-installation/). After that,
 you can simply call `drone exec --local --build-event "pull_request"` within
 your working directory and it will try to run the test suite locally.