add title of 'Supported tags and respective Dockerfile links'

Author: chusiang

README.md

@@ -5,12 +5,14 @@ A Docker image for run [Ansible][ansible_offical] 2.2 on [Jupyter Notebook][jupy
 [ansible_offical]:  https://www.ansible.com/
 [jupyter_official]: http://jupyter.org/
 
-- `alpine-3.4`, `latest` ([alpine/Dockerfile][dockerfile_alpine])
-- `centos-7` ([centos-7/Dockerfile][dockerfile_centos-7])
-- `debian-7` ([debian-7/Dockerfile][dockerfile_debian-7])
-- `debian-8` ([debian-8/Dockerfile][dockerfile_debian-8])
-- `ubuntu-14.04` ([ubuntu-14.04/Dockerfile][dockerfile_ubuntu-14.04])
-- `ubuntu-16.04` ([ubuntu-16.04/Dockerfile][dockerfile_ubuntu-16.04])
+## Supported tags and respective `Dockerfile` links
+
+- `alpine-3.4`, `latest` [*(alpine/Dockerfile)*][dockerfile_alpine]
+- `centos-7` [*(centos-7/Dockerfile)*][dockerfile_centos-7]
+- `debian-7` [*(debian-7/Dockerfile)*][dockerfile_debian-7]
+- `debian-8` [*(debian-8/Dockerfile)*][dockerfile_debian-8]
+- `ubuntu-14.04` [*(ubuntu-14.04/Dockerfile)*][dockerfile_ubuntu-14.04]
+- `ubuntu-16.04` [*(ubuntu-16.04/Dockerfile)*][dockerfile_ubuntu-16.04]
 
 [dockerfile_alpine]:        https://github.com/chusiang/ansible-jupyter.dockerfile/blob/master/alpine/Dockerfile
 [dockerfile_centos-7]:      https://github.com/chusiang/ansible-jupyter.dockerfile/blob/master/centos-7/Dockerfile