The out of tree support section of the nova dev policy
document was pretty old and basically said hooks and
extension points are buyer beware, but doesn't mention
anything about why we don't want them (interoperability)
and the efforts the last few releases to actively
deprecate and remove hooks/extension points/classloading.
This change adds some more wording to that effect to
the doc.
Change-Id: Iaa1cf3ef7ac6e8e1d75f94e17aaba1058474acca
610293fdd5