[Documentation:InstructorUI] add drop lowest description (#560)

add description for drop lowest gradeable.
In future we need to add support from GUI to drop lowest 
Closes Submitty/Submitty/issues/9878

_docs/instructor/course_settings/rainbow_grades/manual_setup.md

@@ -179,3 +179,24 @@ redirect_from:
 
 [SAMPLE_Makefile]: https://github.com/Submitty/RainbowGrades/blob/main/SAMPLE_Makefile
 [SAMPLE_customization.json]: https://github.com/Submitty/RainbowGrades/blob/main/SAMPLE_customization.json
+
+[Drop lowest function]
+Although it is not yet implemented in web-GUI, RainbowGrades support drop lowest grade.
+Directly edit the customization.json file
+Here is a example usage, removing one lowest gradeable from given type
+``` 
+{
+   “count”: 8,
+   “percent”: 0.12,
+   “type”: “quiz”,
+   “remove_lowest”: 1,
+   “ids”: [
+      {
+      “max”: 10.0,
+      “id”: “quiz01",
+      “percent”: 0.125,
+      “released”: true
+      },
+...
+
+