Add vcpkg installation instructions

[PhoebeHui]

Flatbuffers is available as a port in VCPKG , documenting the install process here will help users get started by providing a single set of commands to build flatbuffers, ready to be included in their projects.

VCPKG is a C++ library manager that simplifies installation for flatbuffers and other project dependencies, we also test whether our library ports build in various configurations (dynamic, static) on various platforms (OSX, Linux, Windows: x86, x64, UWP, ARM) to keep a wide coverage for users.

I'm a maintainer for vcpkg, and here is what the port script looks like. We try to keep the library maintained as close as possible to the original library.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

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[PhoebeHui]

@googlebot I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[PhoebeHui]

@googlebot I fixed it.

[PhoebeHui]

@googlebot I fixed it.

[PhoebeHui]

@googlebot I fixed it.

[PhoebeHui]

I'm sure why the cla failed, I signed the cla, also updated my contact email with the account that Github used, additionally configured my gmail on GitHub account.

[aardappel]

Thanks! VCPKG is awesome, hope it becomes the standard for C++, so happy to support :)

Not sure about CLA either..