add contributors to changelog (#58)

CHANGELOG.md

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+# Changelog
+
+## v2.7
+30th April 2018
+
+Notable changes:
+
+* Fix [Issue 43](https://github.com/uswitch/kiam/issues/43): updates to metadata api paths on m5/c5 instances
+* [#41](https://github.com/uswitch/kiam/pull/41): Server allows for custom STS session durations with `--session-duration`
+* Server uses `cache.NewIndexerInformer` to maintain pod and namespace caches, this also addresses an error identified in [Issue 46](https://github.com/uswitch/kiam/issues/46). 
+* [#54](https://github.com/uswitch/kiam/pull/54) Agents can use a `!` prefix on interfaces when configuring iptables rules. This makes it possible to use Kiam with Amazon and Lyft's CNI plugins.
+* Servers will wait for the pod and namespache caches to perform a sync with the Kubernetes API server before accepting gRPC connections. This may cause servers to take longer to start but ensures they have recent state before performing any operations.
+
+Thanks to the following additional people for contributing/helping in this release:
+
+* [elafarge](https://github.com/elafarge)
+* [sami9gag](https://github.com/sami9gag)
+* [mikesplain](https://github.com/mikesplain)
+* [polarbizzle](https://github.com/polarbizzle)
+* [Joseph-Irving](https://github.com/Joseph-Irving)