Add mrbconf documentation.

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+# mruby configuration macros.
+
+## stdio setting.
+`ENABLE_STDIO`
+* Will be defined automatically if `DISABLE_STDIO` isn't defined.
+* Uses `<stdio.h>` functions.
+
+`DISABLE_STDIO`
+* When defined `<stdio.h>` functions won't be used.
+
+## Debug macros.
+`ENABLE_DEBUG`
+* When defined code fetch hook and debug OP hook will be enabled.
+* When using any of the hook set function pointer `code_fetch_hook` and/or `debug_op_hook` of `mrb_state`.
+* Fetch hook will be called before any OP.
+* Debug OP hook will be called when dispatching `OP_DEBUG`.
+
+`DISABLE_DEBUG`
+* Will be define automatically if `ENABLE_DEBUG` isn't defined.
+
+`MRB_DEBUG`
+* When defined `mrb_assert*` macro will be defined with macros from `<assert.h>`.
+* Could be enabled via `enable_debug` method of `MRuby::Build`.
+
+## Stack configuration
+
+`MRB_STACK_EXTEND_DOUBLING`
+* If defined doubles the stack size when extending it.
+* Else extends stack with `MRB_STACK_GROWTH`.
+
+`MRB_STACK_GROWTH`
+* Default value is `128`.
+* Used in stack extending.
+* Ignored when `MRB_STACK_EXTEND_DOUBLING` is defined.
+
+`MRB_STACK_MAX`
+* Default value is `0x40000 - MRB_STACK_GROWTH`.
+* Raises `RuntimeError` when stack size exceeds this value.