Add documentation for pipeline whitelist.

Signed-off-by: Matthew Boedicker <mboedicker@pivotal.io>
Merge pull request #44 from pivotal-cf-experimental/pipeline-whitelist

Add documentation for pipeline whitelist.
Update the developer documentation with information about the pipeline whitelist feature.

Signed-off-by: Kam Leung <kleung@pivotal.io>
Merge pull request #50 from gajwani/master

Update the developer documentation with information about the pipeline w...
Merge pull request #49 from mdaliejaz/master

#47 - Added the missed out image on 'Running out of disk space' page.