Add link to the corresponding Leanpub page

Author: DAXaholic

README.md

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-Despite the title, this is actually a very small, concise book designed to help you understand how PowerShell generates and deals with errors. It's also designed to help you build the best possible error handling for your own scripts and functions, in just a few short lessons.
+Despite the title, this is actually a very small, concise book designed to help you understand how PowerShell generates and deals with errors. It's also designed to help you build the best possible error handling for your own scripts and functions, in just a few short lessons.  
+
+See it on [Leanpub](https://leanpub.com/thebigbookofpowershellerrorhandling)