Update changelog for v1.0.0

Author: valeriobelli

CHANGELOG.md

@@ -0,0 +1,17 @@
+# gh-milestone
+
+## 1.0.0
+
+### Major Changes
+
+- Rename the `states` option of `list` subcommand to `state`. It now takes `all`, `closed` and `open` as possible values.
+  
+  ```bash
+  gh milestone list --state all
+  gh milestone list --state closed
+  gh milestone list --state open
+  ```
+
+### Minor Changes
+
+- Tidy up the `list` subcommand's helper.