Make docu (extensions.rst) stick to convention
Make docu (extensions.rst) stick to convention

The `Conventsions for Community Extensions` say you should name your extension `pyscaffold*ext*.extensionname`, but the documentation uses `pyscaffold.contrib`. If I got it right `contrib` is the extension's name (`$EXTENSION`) and not PEP423's `contrib` which is supposed to be replaced with `ext`. Therefore I changed
`pyscaffold.contrib` to `pyscaffoldext.contrib`.
cpaulik/pyscaffold,blue-yonder/pyscaffold,cpaulik/pyscaffold,blue-yonder/pyscaffold