added changelog file

CHANGELOG.md

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+Unreleased:
+	- Added changelog
+
+*1.3.1*:
+	- Added End Session endpoint
+	- Fixed discovery endpoint
+	- Downgrade MySQL connector dependency version from developer preview to GA release
+
+*1.3.0*:
+	- Added device flow support
+	- Added PKCE support
+	- Modularized UI to allow better overlay and extensions
+	- Modularized data import/export API
+	- Added software statements to dynamic client registration
+	- Added assertion processing framework
+	- Removed ID tokens from storage
+	- Removed structured scopes
+
+*1.2.6*: 
+	- Added string HEART compliance mode