Add docs for examples/folly

[Xuanwo]

@devillove084 spent a lot of time getting folly to work in CI. Would you like to add this to the documentation so our users can use folly on their own?

[Xuanwo]

As far as I know, the docs in README is out-of-date:

You will need a C++ compiler that implements the coroutines TS, which generally coincides with support for C++20. Some C++ compilers (e.g. Apple clang 13.0.0) that implement the coroutines TS crash when compiling Folly. It's also recommended to use libc++ instead of libstdc++, as the former has more complete support for coroutines.

[devillove084]

@Xuanwo Sorry I just noticed that there. No problem. I will reorganize new documents and Readme in the later.