Created FAQ (coding style) (markdown)

FAQ-(coding-style).md

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+= Types
+
+== Why don't you add _t suffix to typedef-ed symbols?
+
+_t suffix is reserved by POSIX.
+Even though We don't express to support POSIX, we can't ignore it.
+
+== I found a symbol without mrb_/MRB_ prefix in the header file.
+
+I'm sorry. It is ill formed. Please report to issue tracking or send pull request.
+
+== My compiler don't support *int*_t. (and/or types may be defined in stdint.h)
+
+It's possible even if your compiler supports C99.
+But it's necessary to obtain the source code portability.
+Please define them by yourself.
+
+Definitions depends on your target architecture and your compiler implementation.
+So it's difficult to give you general guidelines.
+Please consult the compiler vendor.