Docs:add how to use multi-heap region

When we want to use multi-heap region, we should modify two sections.
This document shows them.

docs/HowToUseMultiHeap.md

@@ -0,0 +1,32 @@
+# How to use(add) Multi-heap Regions
+
+There are two cases to use multi-heap regions.  
+1. continuous physical RAM but want to split regions  
+2. separated physical RAMs but want to use for heap  
+
+Modify *[Kconfig](#Kconfig)* and *[TizenRT configuration file](#tizenrt-configuration-file)*.  
+
+## Kconfig
+
+Add **select ARCH_HAVE_HEAPx** belong to config ARCH_ARM in os/arch/<ARCH_NAME>/src/<CHIP_NAME>/Kconfig.
+```
+config ARCH_CHIP_<CHIP_NAME>
+	select ARCH_HAVE_HEAPx
+```
+This will enable HEAPx_BASE and HEAPx_SIZE configs in os/mm/Kconfig.
+
+## TizenRT configuration file
+
+Set base addresses and sizes for new heap regions through menuconfig.
+```
+cd $TIZENRT_BASEDIR
+cd os
+make menuconfig
+```
+Set *CONFIG_HEAPx_BASE* and *CONFIG_HEAPx_SIZE* based on new heap region information.
+```
+Memory Management -> Set List of start address for additional heap region
+Memory Management -> Set List of size for additional heap region
+```
+
+TizenRT will add additional heap region automatically.