Merge pull request #2407 from take-cheeze/mrbconf_update

Update mrbconf document.

doc/mrbconf/README.md

 # mruby configuration macros.
 
+## How to use these macros.
+You can use mrbconfs with following ways:
+* Write them in `mrbconf.h`.
+ * Using compiler flags is prefered  when building a cross binaries or multiple mruby binaries
+ since it's easier to use different mrbconf per each `MRuby::Build`.
+ * Most flags can be enabled by just commenting in.
+* Pass them as compiler flags.
+ * Make sure you pass the same flags to all compilers since some mrbconf(e.g., `MRB_GC_FIXED_ARENA`)
+ changes `struct` layout and cause memory access error when C and other language(e.g., C++) is mixed.
+
 ## stdio setting.
 `ENABLE_STDIO`
 * Will be defined automatically if `DISABLE_STDIO` isn't defined.
@@ -36,3 +46,38 @@
 `MRB_STACK_MAX`
 * Default value is `0x40000 - MRB_STACK_GROWTH`.
 * Raises `RuntimeError` when stack size exceeds this value.
+
+## Primitive type configuration.
+
+`MRB_USE_FLOAT`
+* When defined single precision floating point type(C type `float`) is used as `mrb_float`.
+* Else double precision floating point type(C type `double`) is used as `mrb_float`.
+
+`MRB_INT16`
+* When defined `int16_t` will be defined as `mrb_int`.
+* Conflicts with `MRB_INT64`.
+
+`MRB_INT64`
+* When defined `int64_t` will be defined as `mrb_int`.
+* Conflicts with `MRB_INT16`.
+* When `MRB_INT16` or `MRB_INT64` isn't defined `int`(most of the times 32-bit integer)
+will be defined as `mrb_int`.
+
+## Garbage collector configuration.
+
+`MRB_GC_STRESS`
+* When defined full GC is emitted per each `RBasic` allocation.
+* Mainly used in memory manager debugging.
+
+`MRB_GC_TURN_OFF_GENERATIONAL`
+* When defined turns generational GC by default.
+
+`MRB_GC_FIXED_ARENA`
+* When defined used fixed size GC arena.
+* Raises `RuntimeError` when this is defined and GC arena size exceeds `MRB_GC_ARENA_SIZE`.
+* Useful tracking unnecessary mruby object allocation.
+
+`MRB_GC_ARENA_SIZE`
+* Default value 100.
+* Ignored when `MRB_GC_FIXED_ARENA` isn't defined.
+* Defines fixed GC arena size.