Add link to build command.

README.md

@@ -106,7 +106,7 @@ 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:
+The build command is run from [GitlabCi::Build#command](https://github.com/gitlabhq/gitlab-ci-runner/blob/master/lib/build.rb#L96) 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