Docs: Add section about PMR allocators in C++ documentation

[mikir]

Is your feature request related to a problem? Please describe.
It would be good to have at least some description about usage of PMR allocators in C++ generator.

Describe the solution you'd like
As a first step, it would be enough to create a dedicated section in C++ README.md.

Describe alternatives you've considered
Another option could be to create a dedicated repository with sample of C++ application which will use PMR allocators. However, this will be done only by dedicated request if dedicated section in C++ README.md won't be enough (to save effort).

Additional context
Please consider to use some links to C++17 allocators in a dedicated section for better understanding.