README: coolant flush/valve keys + the valve detent trap (max -> CLOSED)

The 1-5-50-0 cycle's closed detent one press past max is what cooked
both SRM6s in the 2026-07-23 field session -- players need the warning
and the shared-supply note.

players/README.txt

@@ -42,6 +42,12 @@ CONTROLS (keyboard):
   3 or Ctrl ............ upper thumb (missiles) 4 ............ pinky
   V view | B look-behind | M control mode | N schematic | J/K/L MFD pages
   - / = ................ game volume down/up (saved between sessions)
+  H (hold) ............. coolant flush (dumps reservoir to cool everything)
+  C .................... cycle a coolant valve.  CAREFUL: the detents go
+     1 - 5 - 50 (max) - 0 (CLOSED!) - back to 1.  One press past max
+     SHUTS THE LOOP OFF and everything on it will overheat -- watch the
+     valve dial on the coolant screen.  Boosting one loop starves the
+     others (the coolant supply is shared).
   A weapon gone quiet with ammo left has probably JAMMED (heat makes
   ballistic weapons jam -- authentic!).  Check its panel's ENG DATA page
   for the jam lamp (a cocked round with warning bolts).  The pod's fix