Add header image to Readme

Readme.md

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-# Lock
+![Lock](https://phelgo.github.io/assets/github-lock-header.png)
 
 ### Quickly lock your Mac from the Spotlight
 Mac OS X Yosemite brought to Spotlight many features that previously were only available through third-party software [Alfred](http://www.alfredapp.com/). One notorious absence though is the ability to lock the Mac by writing _lock_ in the input field. If you, like me, have been using Alfred for some time it is possible that this shortcut has been hardwired into your brain. The Lock app now provides the same shortcut and can be easily called from the new Spotlight. Bliss.