update README

Signed-off-by: xxchan <xxchan22f@gmail.com>

Author: xxchan

README.md

@@ -7,7 +7,7 @@ Updating a failing test:
 
 https://user-images.githubusercontent.com/1711539/120119633-73b3f100-c1a1-11eb-91be-4c61a23e7060.mp4
 
-Adding a new test:
+Adding a new test: Just leave a blank `expect![[""]]` and update it:
 
 ![expect-fresh](https://user-images.githubusercontent.com/1711539/85926961-306f4500-b8a3-11ea-9369-f2373e327a3f.gif)