Add description and help item for pipeline

Summary

It is not clear what the new feature, called pipeline, is.

Steps to reproduce

1. Open example.com/namespace/project/pipelines. You can see some activity, but it is not clear, what information is collected here (mix of commits and buildstabs or what?)

2. Open example.com/help. There is no pipeline help item available.

Expected behavior

1. On the page example.com/namespace/project/pipelines it would be great to have a short description about what the pipeline is.
2. On the page example.com/help should be a help item where a user can read about the pipeline feature.

Relevant logs and/or screenshots

n/a

Output of checks

n/a

Results of GitLab Application Check

n/a

Results of GitLab Environment Info

System information
System:         Ubuntu 14.04
Current User:   git
Using RVM:      no
Ruby Version:   2.1.8p440   
Gem Version:    2.2.2
Bundler Version:1.9.1
Rake Version:   10.5.0
Sidekiq Version:4.1.2

GitLab information
Version:        8.8.1
Revision:       9b9e320
Directory:      /home/git/gitlab
DB Adapter:     mysql2
URL:            http://example.com
HTTP Clone URL: http://example.com/some-group/some-project.git
SSH Clone URL:  git@example.com:some-group/some-project.git
Using LDAP:     no
Using Omniauth: no

GitLab Shell
Version:        2.7.2
Repositories:   /home/git/repositories/
Hooks:          /home/git/gitlab-shell/hooks/
Git:            /usr/bin/git

Possible fixes

n/a