Add environmental variables to readme.

README.md

@@ -106,6 +106,11 @@ Build script example:
     bundle exec rake db:migrate RAILS_ENV=test
     script/run_all_tests
+The build command is run from GitlabCi::Build#command and contains the following environmental variables:
+
+    CI_SERVER, CI_SERVER_NAME, CI_SERVER_VERSION, CI_SERVER_REVISION
+    CI_BUILD_REF, CI_BUILD_BEFORE_SHA, CI_BUILD_REF_NAME, CI_BUILD_ID
+
 ### Getting help
 * [Feedback and suggestions forum](http://feedback.gitlab.com/forums/176466-general/category/64310-gitlab-ci) is the place to propose and discuss new features for GitLab CI.