modify contents of libraries's README
modify legacy explanations and add a method to add external libraries
-lib/ README File
-================
+build/output/libraries/ README File
+===================================
-This directory is reserved for libraries generated during the NuttX build process
+This directory is reserved for libraries generated during the TinyAra build process
+How to add external libraries
+=============================
+If you want to add pre-built libraries into TinyAra binary,
+please refer belows.
+
+1. place your libraries at build/output/libraries folder
+2. modify mk files to add them to binary
+
+ < Files >
+ FlatLibs.mk for flat build
+ ProtectedLibs.mk for protected build
+ KernelLibs.mk for kernel build
+
+ < Contents to modify >
+ TINYARALIBS += $(LIBRARIES_DIR)$(DELIM)<File name of Library>$(LIBEXT)
+ or
+ USERLIBS += $(LIBRARIES_DIR)$(DELIM)<File name of Library>$(LIBEXT)
+ You should replace <File name of Library> to yours.
+
+3. build the TinyAra
