[open-data-manual] Guide to contributing revised

[Tim McNamara]

Hi all,

I've expanded several wiki edits from Rufus outlining a few roles that
exist within the project
(http://wiki.okfn.org/Open_Data_Manual#Contributing). If you get a
chance, I would welcome people to review them and see whether they
make sense. Most importantly, I would like to know whether they answer
'how do I get started?' for people new to the project.