docs: Update docs to reflect newer workflow.

The workflow has recently changed such that all development is done on the 'os-build' branch. Update the docs to show how easy it is to make a change, commit it, generate the srpm and upload it to koji.

Also add it a build dep for making a srpm: patchutils (for filterdiff).

Resolves: #33 (closed)

Cc: Bastien Nocera bnocera@redhat.com Cc: Prarit Bhargava prarit@redhat.com Cc: Justin Forbes jforbes@redhat.com Signed-off-by: Don Zickus dzickus@redhat.com