good/bad examples of adding book entries

CONTRIBUTING.md

@@ -42,8 +42,14 @@ Like this example:
 * [IllumOS Dynamic Tracing Guide](http://dtrace.org/guide/preface.html)
 * [Some Other Book](http://so.me/other/book.html)
 
+BAD : * [IllumOS Dynamic Tracing Guide](http://dtrace.org/guide/preface.html)(PDF)
+GOOD: * [IllumOS Dynamic Tracing Guide](http://dtrace.org/guide/preface.html) (PDF)
+
+BAD : * [IllumOS Dynamic Tracing Guide](http://dtrace.org/guide/preface.html)- Robert
+GOOD: * [IllumOS Dynamic Tracing Guide](http://dtrace.org/guide/preface.html) - Robert
 ```
 
+
 And that's it! Simple, easy and lovely...
 
 We hope you contribute to this great repository. :+1: