Update changelog

Signed-off-by: Ying Li <ying.li@docker.com>

Author: cyli

CHANGELOG.md

@@ -5,6 +5,7 @@
 + Output message to CLI when repo changes have been successfully published [#974](https://github.com/docker/notary/pull/974)
 + Improved error messages for client authentication errors and for the witness command [#972](https://github.com/docker/notary/pull/972)
 + Support for finding keys that are anywhere in the notary directory's "private" directory, not just under "private/root_keys" or "private/tuf_keys" [#981](https://github.com/docker/notary/pull/981)
++ Previously, on any error updating, the client would fall back on the cache.  Now we only do so if there is a network error or if the server is unavailable or missing the TUF data. Invalid TUF data will cause the update to fail - for example if there was an invalid root rotation. [#982](https://github.com/docker/notary/pull/982)
 
 ## [v0.4.0](https://github.com/docker/notary/releases/tag/v0.4.0) 9/21/2016
 + Server-managed key rotations [#889](https://github.com/docker/notary/pull/889)