Clarify purpose of tests/screenshots

Author: Xing Han Lu

tests/screenshots/readme.md

@@ -1,3 +1,3 @@
-Please do not add unecessary files to this directory. This is reserved for auto-generated screenshots from testing. 
+This is directory is reserved for screenshots generated by Selenium's webdriver in `test_usage.py`, during the CircleCI builds.
 
-Please do not move this file.
+Please do not add unecessary files to this directory (in fact, the only file should be this `readme.md`), and do not move this file.