Update build.md

Add information about downloading the Skia native libraries via the supplied script.

docs/build.md

@@ -18,6 +18,8 @@ We currently need to build our own private version of ReactiveUI as it doesn't w
 is linked as a submodule in the git repository, so run:
 
     git submodule update --init
+    
+The next step is to download the Skia native libraries. Run ```getnatives.ps1``` PowerShell script which can be found under the folder ```Perspex\src\Skia\```.
 
 ## Linux
 
@@ -43,6 +45,8 @@ We currently need to build our own private version of ReactiveUI as it doesn't w
 is linked as a submodule in the git repository, so run:
 
     git submodule update --init
+    
+The next step is to download the Skia native libraries. Run ```getnatives.sh``` script which can be found under the folder ```Perspex\src\Skia\```.
    
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