Docs: add how to add static library

When we want to include static (built) library, there are two methods.
This document shows them.

docs/HowToAddStaticLibrary.md

@@ -0,0 +1,55 @@
+# How to add Static Library
+
+There are two methods to include static library, [adding it into arch library](#adding-it-into-arch-library) and [adding it as a new library](#adding-it-as-a-new-library).
+
+## Adding it into arch library
+
+Makefile or Make.defs in arch can include it for architecture specific library as mentioned below.  
+Libarch.a includes new static library inside.
+```
+VPATH += <LIB_PATH>
+EXTRA_LIBS += <LIB_PATH>/<LIB_NAME>.a
+```
+The *LIB_PATH* should be a relative path from *os/arch/arm/src*.
+
+For example,
+```
+VPATH += chip/abc
+EXTRA_LIBS += chip/abc/libnew.a
+```
+
+This change makes it add to Tizen RT binary by Makefile which is at *os/arch/arm/src* folder.
+```
+$(BIN_DIR)/tinyara$(EXEEXT): $(HEAD_OBJ) board/libboard$(LIBEXT)
+	$(Q) echo "LD: tinyara"
+	$(Q) $(LD) --entry=__start $(LDFLAGS) $(LIBPATHS) $(EXTRA_LIBPATHS) \
+		-o $(TINYARA) $(HEAD_OBJ) $(EXTRA_OBJS) \
+		--start-group $(LDLIBS) $(EXTRA_LIBS) $(LIBGCC) --end-group -Map $(TOPDIR)/../build/output/bin/tinyara.map
+```
+
+## Adding it as a new library
+
+Tizen RT includes it as separated library, not merge it to existed library.
+
+1. Add it in *Libtargets.mk*.  
+    ```
+    $(LIBRARIES_DIR)$(DELIM)<LIB_NAME>$(LIBEXT): <LIB_PATH>$(DELIM)<LIB_NAME>$(LIBEXT)
+    	$(Q) install <LIB_PATH>$(DELIM)<LIB_NAME>$(LIBEXT) $(LIBRARIES_DIR)$(DELIM)<LIB_NAME>$(LIBEXT)
+    ```
+
+2. Add it in *FlatLibs.mk*, *ProtectedLibs.mk* and *KernelLibs.mk*.
+
+    For kernel library,
+    ```
+    TINYARALIBS += $(LIBRARIES_DIR)$(DELIM)<LIB_NAME>$(LIBEXT)
+    ```
+
+    For user library,
+    ```
+    USERLIBS += $(LIBRARIES_DIR)$(DELIM)<LIB_NAME>$(LIBEXT)
+    ```
+
+    In flat build, there is no difference between TINYARALIBS and USERLIBS.  
+    But in protected build and kernel build, Tizen RT splits kernel space and user space. So, new static library should be included at appropriate space.
+
+The *LIB_PATH* should be a relative path from *os*.

docs/Porting_Guide.md

@@ -15,7 +15,7 @@ Don't call architecture codes from application directly. When protected build en
 ## Kernel & System
 1. [How to add new board](HowToAddnewBoard.md)
 2. How to use peripheral (will be updated)
-3. How to add static library (will be updated)
+3. [How to add static library](HowToAddStaticLibrary.md)
 
 ## File System
 Will be updated