docs: update Changelog

Author: ericwa

changelog.txt

@@ -1,10 +1,12 @@
-2018-03-23 ericw-tools 0.18.1
+2018-04-05 ericw-tools 0.18.1
 
 - qbsp: fix crash when worldspawn has 0 brushes
 - qbsp: support reading Q2/Q3 detail flag
 - qbsp: experimental "_noclipfaces" key
 - qbsp: fix "_mirrorinside" on bmodels
+- qbsp: improve an error message for when BSP2 is needed
 - vis: fix "average leafs visible" message overflowing
+- light: fix crash with surface lights
 
 2018-02-18 ericw-tools 0.18