Add CHANGELOG to dynamodb encryption client for java

This starts a changelog, which follows the same format as the encryption sdk
for java.

Author: johnwalker

CHANGELOG.md

@@ -0,0 +1,10 @@
+# Changelog
+
+## 1.2.0
+
+### Minor Changes
+
+* Add support for overriding the EncryptionContext in DynamoDBEncryptor #60
+* Update examples to use PUT instead of Clobber #60
+* Document the minimum supported JDK version as JDK8 #57
+* Add CI for JDK11 #62