Add examples to badges docs

We need to add some examples in https://docs.gitlab.com/ee/user/project/badges.html. https://shields.io/ is a good start.

• https://docs.gitlab.com/ee/user/project/badges.html#comment-3867752346
• https://about.gitlab.com/2018/04/22/gitlab-10-7-released/#comment-3869810347

Internal discussion: https://gitlab.slack.com/archives/C16HYA2P5/p1525428525000104