Add changelog

CHANGELOG.md

+# Changelog for kea-hook-runscript
+
+## 1.1.0
+
+- Add lots of DHCPv6 variables: DHCPv6 should now be usable
+- Add some more DHCPv4 variables, especially for leases
+- Safety fixes: fix potential NULL pointer dereference, and unsafe string usage
+- Makefile: use standard CXX variable instead of hard-coding g++
+
+## 1.0.0
+
+- Initial release