README.md: Use the vcpkg submodule instead of separately cloning

[StephanTLavavej]

Currently, the README depicts separately cloning vcpkg; these instructions were originally added by #8 on Sep 6, 2019. When #114 added vcpkg as a submodule on Sep 25, 2019, we didn't update the instructions.

I believe that we should overhaul our instructions to use the submodule. That should avoid confusion/redundancy, and should ensure that known-compatible versions of vcpkg and Boost are used.

[StephanTLavavej]

We can mention "or you can use a separate installation of vcpkg".

[BillyONeal]

I think this should only change the command line instructions rather than the IDE instructions, as the IDE instructions depend on integrate install which is a global change so it should not use a project-specific clone of vcpkg.