Update CHANGES.txt

Author: pyahmed

pycorn/docs/CHANGES.txt

@@ -1,3 +1,7 @@
+v0.18
+======
+- fixed xmin being ignored when 0
+
 v0.17
 ======
 - Added experimental support for extracting and plotting UNICORN6 zip bundles
@@ -50,4 +54,4 @@ v0.11
 
 v0.1
 ======
-- Initial release
+- Initial release