add staged flag for clarity

README.md

@@ -5,7 +5,7 @@
 Git plugin that helps prevent sensitive data from being committed by sniffing potential commits against regular expressions from a local `.githound.yml` file.
 
 ## How does it work?
-Upon commit, it runs the output of `git diff -U0` through the Hound, which matches every _added_ or _modified_ line against your provided list of regular expressions. This runs in O(m*n) time (where m is the number of lines and n is the number of patterns), so be sure to commit often. But you should be doing that anyway, right?
+Upon commit, it runs the output of `git diff -U0 --staged` through the Hound, which matches every _added_ or _modified_ line against your provided list of regular expressions. This runs in O(m*n) time (where m is the number of lines and n is the number of patterns), so be sure to commit often. But you should be doing that anyway, right?
 
 ## Installation
 To install Hound, please use `go get`. If you don't have Go installed, [get it here](https://golang.org/dl/). If you would like to grab a precompiled binary, head over to the [releases](https://github.com/ezekg/git-hound/releases) page. The precompiled Hound binaries have no external dependencies.