Add a CHANGELOG

CHANGELOG.md

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+# HCL Changelog
+
+## v2.0.0 (Unreleased)
+
+Initial release of HCL 2, which is a new implementating combining the HCL 1
+language with the HIL expression language to produce a single language
+supporting both nested configuration structures and arbitrary expressions.
+
+HCL 2 has an entirely new Go library API and so is not a drop-in upgrade
+relative to HCL 1. It's possible to import both versions of HCL into a single
+program using Go's _semantic import versioning_ mechanism:
+
+```
+import (
+    hcl1 "github.com/hashicorp/hcl"
+    hcl2 "github.com/hashicorp/hcl/v2"
+)
+```
+
+---
+
+Prior to v2.0.0 there was not a curated changelog. Consult the git history for
+information on the changes to HCL 1.