add example except/variables

[ondrejsika]

# .gitlab-ci.yml

stages:
  - build
  - test
  - deploy

build:
  stage: build
  script: echo build && sleep 10
  except:
    variables:
      - $SKIP_STAGE_build == "1" && $SKIP_BRANCH == $CI_COMMIT_REF_NAME

test:
  stage: test
  script: echo test && sleep 10
  except:
    variables:
      - $SKIP_STAGE_test == "1" && $SKIP_BRANCH == $CI_COMMIT_REF_NAME

deploy:
  stage: deploy
  script: echo deploy
  except:
    variables:
      - $SKIP_STAGE_deploy == "1" && $SKIP_BRANCH == $CI_COMMIT_REF_NAME