docs: Focus on testing

Unfortunately, because of skeptic, we can only adjust the Cargo.toml
line.

Fixes #62

Author: epage

README.md

@@ -6,10 +6,10 @@
 
 ## Install
 
-Just add it to your `Cargo.toml`:
+For your tests, just add it to your `Cargo.toml`:
 
 ```toml
-[dependencies]
+[dev-dependencies]
 assert_cli = "0.5"
 ```