Add contributors to chart changelog
- Make SSH known hosts volume optional
[weaveworks/flux#1544](https://github.com/weaveworks/flux/pull/1544)
+### Thanks
+
+Thanks to @davidkarlsen, @stephenmoloney, @batpok, @squaremo,
+@hiddeco and @stefanprodan for their contributions.
+
## 0.5.1 (2018-11-21)
### Bug fixes
