cosmetic: README: add some entries to Directory Hierarchy

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/README b/README
index 16fdd37..f91e0442 100644
--- a/README
+++ b/README
@@ -132,6 +132,10 @@
 ====================
 
 /arch			Architecture specific files
+  /arc			Files generic to ARC architecture
+    /cpu		CPU specific files
+      /arc700		Files specific to ARC 700 CPUs
+    /lib		Architecture specific library files
   /arm			Files generic to ARM architecture
     /cpu		CPU specific files
       /arm720t		Files specific to ARM 720 CPUs
@@ -164,6 +168,7 @@
   /mips			Files generic to MIPS architecture
     /cpu		CPU specific files
       /mips32		Files specific to MIPS32 CPUs
+      /mips64		Files specific to MIPS64 CPUs
     /lib		Architecture specific library files
   /nds32		Files generic to NDS32 architecture
     /cpu		CPU specific files