Documentation: Simple switching between different vcpkg instances, or tags of your custom library

[Fleischner]

We use a very simple system to tag our vcpkg custom build library:

• As vcpkg internally generates build artifacts with global paths we use always the same global vcpkg 'work directory' and switch to different library versions by using windows hard links.
• Tagging is done by coping the whole vcpkg directory (either with complete build-tree, or cleaned up to save space)

As a sample, we use the following directory structure for our vcpkg:
c:\code\vcpkg <- this is the work directory and a hard-link to one of the following directories:
c:\code\vcpkg_trunk <- current development branch
c:\code\vcpkg_v201703 <- tagged library with time stamp
c:\code\vcpkg_...

Each tagged library version has an batch file to set it as active library:

LinkAsActive.bat:
#Uncomment and edit the following line as needed.
#mklink /J c:\code\vcpkg c:\code\vcpkg_...

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Missing vcpkg features:

• Easy way to restore build-tree with all source directories (source code is needed for debugging)