Merge bitcoin#17688: doc: Add "ci" prefix to CONTRIBUTING.md

19267cb doc: Add ci prefix to CONTRIBUTING.md (Hennadii Stepanov)

Pull request description:

  It seems our maintainers like `ci` prefix for commits and PRs:
  ```
  git log | grep 'ci:'
  ```

  and
  ![Screenshot from 2019-12-07 11-49-51](https://user-images.githubusercontent.com/32963518/70372457-ec592a80-18e7-11ea-9320-73412a1ccd25.png)

  So let's document it.

ACKs for top commit:
  fanquake:
    ACK 19267cb - this looks ok

Tree-SHA512: ef467513f9562f51d0471c5cc900336caab4e9494299bcd2c9cc9e1b296536a86467807d71b8d7100a5c78715174cf58b6ecfe6c3bd958060c15eba4fba7067f

CONTRIBUTING.md

@@ -88,10 +88,10 @@ the pull request affects. Valid areas as:
   - `refactor` for structural changes that do not change behavior
   - `rpc`, `rest` or `zmq` for changes to the RPC, REST or ZMQ APIs
   - `script` for changes to the scripts and tools
-  - `test` for changes to the bitcoin unit tests or QA tests
+  - `test`, `qa` or `ci` for changes to the unit tests, QA tests or CI code
   - `util` or `lib` for changes to the utils or libraries
   - `wallet` for changes to the wallet code
-  - `build` for changes to the GNU Autotools, reproducible builds or CI code
+  - `build` for changes to the GNU Autotools or reproducible builds
 
 Examples: