docs: inline wiki page Refactoring.md into MAINTAIN.md

MAINTAIN.md

@@ -168,6 +168,22 @@ Non-technical dependencies
     * pkgjson.org
 * DNS for the above domains is managed in https://cloudflare.com (not the domain registrar)
 
+
+Refactoring
+-----------
+
+### Frozen legacy modules
+
+Refactoring Vim structurally and aesthetically is an important goal of Neovim.
+But there are some modules that should not be changed significantly, because
+they are maintained Vim, at present. Until someone takes "ownership" of these
+modules, the cost of any significant changes (including style or structural
+changes that re-arrange the code) to these modules outweighs the benefit. The
+modules are:
+
+- `regexp.c`
+- `indent_c.c`
+
 Automation (CI)
 ---------------