Update on Gatsby Windows instructions (#1216)

After solving this myself

docs/docs/gatsby-on-windows.md

@@ -16,3 +16,27 @@ powershell console. On installing this package, it downloads and installs the
 Visual C++ Build Tools 2015, provided free of charge by Microsoft. These tools
 are required to compile popular native modules. It will also install Python
 2.7, configuring your machine and npm appropriately.
+
+### If build or `npm i` fails you might want to try this
+
+Sometimes the `windows-build-tools` won't properly install required librairies. This is true if you already have a regular .Net dev environment setup. This has been reported on Windows 10 x64 (and possibliy other arch or Windows versions). 
+
+You can identify the problem after running `npm i` on Gatsby, showing compilation errors with `node-gyp` or `sharp` or `binding.gyp not found`.
+
+In that case, download the [Visual Studio Express 2015 Package](https://www.visualstudio.com/products/visual-studio-community-vs) and install the package `Programming languages > Visual C++ > Common tools for Visual Studio 2015`. You can uncheck everything else. You don't need to install the full VS2015 Express on your system and this won't mess up your existing VS201x installs.
+
+Then run the commands on Gatsby: 
+
+`npm uninstall node-gyp -g`
+
+`npm config set python python2.7`
+
+`npm config set msvs_version 2015`
+
+`npm cache clean -f`
+
+`npm i`
+
+You should then be all set. 
+
+If that still doesn't work, refer to the [`node-gyp` npm package homepage](https://www.npmjs.com/package/node-gyp) for further instructions and contact the `node-gyp`team on [GitHub](https://github.com/nodejs/node-gyp/issues).