Merge pull request progit#1599 from bagasme/update-git-credential-ro-usage

Update usage snippet and code explanation for git credential-read-only

Author: ben

book/07-git-tools/sections/credentials.asc

@@ -174,7 +174,7 @@ include::../git-credential-read-only[]
 <3> This loop reads from stdin until the first blank line is reached.
     The inputs are stored in the `known` hash for later reference.
 <4> This loop reads the contents of the storage file, looking for matches.
-    If the protocol and host from `known` match this line, the program prints the results to stdout and exits.
+    If the protocol, host, and username from `known` match this line, the program prints the results to stdout and exits.
 
 We'll save our helper as `git-credential-read-only`, put it somewhere in our `PATH` and mark it executable.
 Here's what an interactive session looks like:
@@ -184,6 +184,7 @@ Here's what an interactive session looks like:
 $ git credential-read-only --file=/mnt/shared/creds get
 protocol=https
 host=mygithost
+username=bob
 
 protocol=https
 host=mygithost