Update README documentation

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3543 42af7a65-404d-4744-a932-0658087f49c3

README.txt

@@ -415,6 +415,9 @@ Below is a guide to the available README files in the NuttX source tree:
  |- examples/
  |   |- pashello/README.txt
  |   `- README.txt
+ |- fs/
+ |   `- nxffs/
+ |       `- README.txt
  |- graphics/
  |   `- README.txt
  |- lib/

fs/nxffs/README.txt

+NXFFS README
+^^^^^^^^^^^^
+
+This README file contains information about the implemenation of the NuttX
+wear-leveling FLASH file system, NXFFS.
+
+Contents:
+
+  General NXFFS organization
+  General operation
+  Headers
+  NXFFS Limitations
+
 General NXFFS organization
 ==========================