Provide instructions to use run-ci to build a single plan

When testing the behavior of CI, I had difficulties to convince run-ci to build only the plan for a package of interest.

Among the difficulties, the env var ONLY_PACKAGES seemed to be ignored, and calls to extra_package had to be removed in ci/config.sh, and the BUILD_MODE env var had to be set to FULL to skip the tests.

When possible, these concerns need to be solved for users, or otherwise, the README could explain how to invoke run-ci for this use case.