record another known issue I don't know how to fix yet

Dependencies

• [2] IDGP4BJZ new known issue with drawings
• [*] FS2ITYYH record a known issue
• [*] 242L3OQX bugfix: ensure Cursor_line is always on a text line

Change contents

• edit in README.md at line 15
  [2.15]

  [5.1190]
  + * Large files may grow sluggish in other ways. I've noticed in 100KB files

  + that closing the window can take a few seconds. And it seems to take longer

  + in proportion to how far down my edits are. The phenomenon persists even if

  + I take out undo history.

  +