#8 install doc

gitlab-ci-runner seems to be less document project, so here are some updates I've made while installing fresh runner today on Centos 6.5.

CHANGELOG

v5.2.0
- Add RUNNER_DESCRIPTION and RUNNER_TAG_LIST that can be send during runner's registration
+  - Updated installation documentation
v5.1.0
- Add `-C` command-line option to specify the working directory