feat(README): add note about the license

README.md

@@ -467,3 +467,9 @@ curl -L http://127.0.0.1:4001
 During the v0 series of releases we may break the API as we fix bugs and get feedback.
 
 [semver]: http://semver.org/
+
+### License
+
+etcd is under the Apache 2.0 license. See the [LICENSE][license] file for details.
+
+[license]: https://github.com/coreos/etcd/blob/master/LICENSE