README: Fix hyphenation in the directory docs

Hyphens are missing in various places where the intent is to create an
adjective. Fix it.
Signed-off-by: Simon Glass <sjg@chromium.org>

README

@@ -128,7 +128,7 @@ Examples:
 Directory Hierarchy:
 ====================
 
-/arch			Architecture specific files
+/arch			Architecture-specific files
   /arc			Files generic to ARC architecture
   /arm			Files generic to ARM architecture
   /m68k			Files generic to m68k architecture
@@ -142,16 +142,16 @@ Directory Hierarchy:
   /sh			Files generic to SH architecture
   /x86			Files generic to x86 architecture
   /xtensa		Files generic to Xtensa architecture
-/api			Machine/arch independent API for external apps
-/board			Board dependent files
+/api			Machine/arch-independent API for external apps
+/board			Board-dependent files
 /cmd			U-Boot commands functions
-/common			Misc architecture independent functions
+/common			Misc architecture-independent functions
 /configs		Board default configuration files
 /disk			Code for disk drive partition handling
-/doc			Documentation (don't expect too much)
-/drivers		Commonly used device drivers
-/dts			Contains Makefile for building internal U-Boot fdt.
-/env			Environment files
+/doc			Documentation (a mix of ReST and READMEs)
+/drivers		Device drivers
+/dts			Makefile for building internal U-Boot fdt.
+/env			Environment support
 /examples		Example code for standalone applications, etc.
 /fs			Filesystem code (cramfs, ext2, jffs2, etc.)
 /include		Header Files
@@ -161,7 +161,7 @@ Directory Hierarchy:
 /post			Power On Self Test
 /scripts		Various build scripts and Makefiles
 /test			Various unit test files
-/tools			Tools to build S-Record or U-Boot images, etc.
+/tools			Tools to build and sign FIT images, etc.
 
 Software Configuration:
 =======================