Add documentation of `mrb_value` config.

doc/mrbconf/README.md

@@ -104,3 +104,18 @@ largest value of required alignment.
 `MRB_FIXED_STATE_ATEXIT_STACK_SIZE`
 * Default value is `5`.
 * If `MRB_FIXED_STATE_ATEXIT_STACK` isn't defined this macro is ignored.
+
+## `mrb_value` configuration.
+
+`MRB_ENDIAN_BIG`
+* If defined compiles mruby for big endian machines.
+* Used in `MRB_NAN_BOXING`.
+* Some mrbgem use this mrbconf.
+
+`MRB_NAN_BOXING`
+* If defined represent `mrb_value` in boxed `double`.
+* Conflicts with `MRB_USE_FLOAT`.
+
+`MRB_WORD_BOXING`
+* If defined represent `mrb_value` as a word.
+* If defined `Float` will be a mruby object with `RBasic`.