Document that on Windows the library, the plugins and any code that uses them should be compiled with ABI-compatible compilers and link the same CRT

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This may be obvious for experienced Windows developers, but could be helpful for beginners.
See https://www.reddit.com/r/cpp/comments/9c9kqz/do_cpp_users_wantneed_a_cross_platform_framework/e5bxk6o .