update CHANGELOG.md for #2989

Author: bhcleek

CHANGELOG.md

@@ -58,6 +58,8 @@ IMPROVEMENTS:
   [[GH-2983]](https://github.com/fatih/vim-go/pull/2983)
 * Clear highlighting of the current line when after resuming when debugging.
   [[GH-2984]](https://github.com/fatih/vim-go/pull/2984)
+* Add `:GoDebugAttach` to debug a running process.
+  [[GH-2989]](https://github.com/fatih/vim-go/pull/2989)
 
 BUG FIXES:
 * Fix call to non-existent function in terminal mode edge case.