add a changelog to track changes (#303)

* add a changelog to track changes

* Update CHANGELOG.md

Co-authored-by: Callum Waters <cmwaters19@gmail.com>

Co-authored-by: Callum Waters <cmwaters19@gmail.com>

CHANGELOG.md

@@ -0,0 +1,7 @@
+# Changelog
+
+All notable changes to the spec will be documented in this file for easy reference. Changes are grouped by version.
+
+## [Unreleased]
+
+- [RPC] [#276](https://github.com/tendermint/spec/pull/276): Add specification for RPC (@marbar3778)