Update changelog to include recent changes

ChangeLog

@@ -1,3 +1,17 @@
+What's new in version 3.1.2
+
+* Bugfix for crash when storing modified settings at exit
+* Allow -u UID with numerical value as argument
+* Elaborate documentation for obsolete/state libraries/program files highlighting
+* Some obsolete/stale library highlighting refinements
+* Column width issues
+* Dynamic UID column sizing
+* Discard stale information from Disk and Network I/O meters
+* Refined Linux kernel thread detection
+* Reworked prcess state handling
+* New CCGROUP column showing abbreviated cgroup name
+* New OFFSET column in the list of open files screen
+
 What's new in version 3.1.1
 
 * Update license headers to explicitly say GPLv2+