add comment to rustdoc.css on updating filename suffixes

src/librustdoc/html/static/css/rustdoc.css

@@ -1,3 +1,11 @@
+/* When static files are updated, their suffixes need to be updated.
+	1. In the top directory run:
+		./x.py doc --stage 1 library/core
+	2. Find the directory containing files named with updated suffixes:
+		find build -path '*'/stage1-std/'*'/static.files
+	3. Copy the filenames with updated suffixes from the directory.
+*/
+
 /* See FiraSans-LICENSE.txt for the Fira Sans license. */
 @font-face {
 	font-family: 'Fira Sans';