added item to release checklist for updating docs repo (elastic#1109)

Author: beniwohli

CONTRIBUTING.md

@@ -180,6 +180,7 @@ If you have commit access, the process is as follows:
 1. Update the version in `elasticapm/version.py` according to the scale of the change. (major, minor or patch)
 1. Update `CHANGELOG.asciidoc`. Rename the `Unreleased` section to the correct version (`vX.X.X`), and nest under the appropriate sub-heading, e.g., `Python Agent version 5.x`.
 1. For Majors: Add a new row to the EOL table in `docs/upgrading.asciidoc`. The EOL date is the release date plus 18 months.
+1. For Majors: Add the new major version to `conf.yaml` in the [elastic/docs](https://github.com/elastic/docs) repo. 
 1. Commit changes with message `update CHANGELOG and bump version to X.Y.Z` where `X.Y.Z` is the version in `elasticapm/version.py`
 1. Tag the commit with `git tag -a vX.Y.Z`, for example `git tag -a v1.2.3`.
    Copy the changelog for the release to the tag message, removing any leading `#`.