Update Visual Studio Integration Docs

[rioki]

As per the last comment in #18 there is a less intrusive way to integrate clcache.

Each vcxproj file should include "$(UserRootDir)\Microsoft.Cpp.$(Platform).user.props" In this props file you can add the following snipet (when installed with chocolatey):

<PropertyGroup>
    <CLToolExe>clcache.exe</CLToolExe>
    <CLToolPath>C:\ProgramData\chocolatey\bin\</CLToolPath>
    <TrackFileAccess>false</TrackFileAccess>
</PropertyGroup> 

The will neither mean you need to tamper with your Visual Studio installation or your project files.

It would be nice, if the documentation can be updated accordingly.