[ANN] Credentials plugin documentation

[Stephen Connolly]

I have finally finished documenting the credentials plugin.

You can find the documentation at https://github.com/jenkinsci/credentials-plugin/tree/master/docs

I have also uploaded PDF versions of the documentation to the plugin's wiki page: https://wiki.jenkins.io/display/JENKINS/Credentials+Plugin

Specifically many people are probably looking for the user guide:  https://wiki.jenkins.io/download/attachments/59511751/user.pdf

The definitive version of the documentation is always the GitHub version, though due to formatting issues in the GitHub Asciidoc preview, the PDFs can be easier to follow.

This is essentially a brain-dump by me, so pull requests are most welcome...

-Stephen