Added note about why Binaries folder isn't included.

source/project/unreal_clean.py

@@ -27,6 +27,11 @@ ignore_folders = {
 # NOTE: These are recursively deleted.
 dirs_to_delete = {
     ".vs",
+    # The Binaries folder commonly contains checked-in files. This script
+    # isn't designed to check against version control, it's supposed to 
+    # be a quick simple script to quickly get back disk space. Uncomment
+    # this folder at your own peril.
+    #"Binaries",
     "DerivedDataCache",
     "Intermediate",
     "Saved",