Update CONTRIBUTING.md

guidelines for adding multiple links to the same book

CONTRIBUTING.md

@@ -53,6 +53,10 @@ GOOD: * [IllumOS Dynamic Tracing Guide](http://dtrace.org/guide/preface.html) -
 
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+#### What to do about multiple links to the same book ([#1192](https://github.com/vhf/free-programming-books/issues/1192#issuecomment-135969100))
++ if its a different edition, add separate link with a note about its edition
++ if its the same book but in a better format, replace link
++ if one is a webpage and the other is pdf, add separate link with a note about its format
 
 And that's it! Simple, easy and lovely...