Update instructions to add SHA-1 hash

README.md

@@ -66,7 +66,8 @@ Fork/clone the [GitHub repository created for this
 assignment](http://github.com/rdpeng/RepData_PeerAssessment1). You
 will submit this assignment by pushing your completed files into your
 forked repository on GitHub. The assignment submission will consist of
-the URL to your GitHub repository.
+the URL to your GitHub repository and the SHA-1 commit ID for your
+repository state.
 
 NOTE: The GitHub repository also contains the dataset for the
 assignment so you do not have to download the data separately.
@@ -145,3 +146,23 @@ To submit the assignment:
 4. Push your `master` branch to GitHub.
 
 5. Submit the URL to your GitHub repository for this assignment on the course web site.
+
+In addition to submitting the URL for your GitHub repository, you will
+need to submit the 40 character SHA-1 hash (as string of numbers from
+0-9 and letters from a-f) that identifies the repository commit that
+contains the version of the files you want to submit. You can do this
+in GitHub by doing the following
+
+1. Going to your GitHub repository web page for this assignment
+
+2. Click on the "?? commits" link where ?? is the number of commits you have in the repository. For example, if you made a total of 10 commits to this repository, the link should say "10 commits".
+
+3. You will see a list of commits that you have made to this repository. The most recent commit is at the very top. If this represents the version of the files you want to submit, then just click the "copy to clipboard" button on the right hand side that should appear when you hover over the SHA-1 hash. Paste this SHA-1 hash into the course web site when you submit your assignment. If you don't want to use the most recent commit, then go down and find the commit you want and copy the SHA-1 hash.
+
+A valid submission will look something like (this is just an **example**!)
+
+```r
+https://github.com/rdpeng/RepData_PeerAssessment1
+
+7c376cc5447f11537f8740af8e07d6facc3d9645
+```