Documentation on CI triggers

Closes #3432 (closed)

Notes

• Implement build trigger API - https://gitlab.com/gitlab-org/gitlab-ci/merge_requests/229
• Build trigger API - https://gitlab.com/gitlab-org/gitlab-ci/issues/257
• Build pipeline - https://dev.gitlab.org/gitlab/gitlab-ci/issues/282 and https://gitlab.com/gitlab-org/gitlab-ce/issues/3743
• Dependent builds - https://dev.gitlab.org/gitlab/gitlab-ci/issues/328
• Travis docs - https://docs.travis-ci.com/user/triggering-builds/
• Custom variables Circle CI example - https://circleci.com/docs/nightly-builds
• Triggers API (CI) will be done in a separate MR
• Notify https://gitlab.com/gitlab-org/gitlab-ci/issues/368 once done

Docs needed to change

• doc/ci/README.md
• doc/README.md