Explain how to add custom platform flavors for C/C++
Explain how to add custom platform flavors for C/C++

Summary:
- Explain how to create a custom platform flavor for a C/C++ target. We already have this documented for Python; do the same for C/C++.
- Link to Buck sample for C++ on GitHub which demos this functionality.

Reviewed By: styurin

fbshipit-source-id: fd3ad9fb58

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