Fix zipalign README file and usage strings.

Addresses comments from change 13e5965306.

Change-Id: I0a2c0048781858d3aa06347a77220547cfcdcebe
diff --git a/tools/zipalign/README.txt b/tools/zipalign/README.txt
index 9c7d07e..0b80b35 100644
--- a/tools/zipalign/README.txt
+++ b/tools/zipalign/README.txt
@@ -5,6 +5,7 @@
 
   -c : check alignment only (does not modify file)
   -f : overwrite existing outfile.zip
+  -p : page align stored shared object files
   -v : verbose output
   <align> is in bytes, e.g. "4" provides 32-bit alignment
   infile.zip is an existing Zip archive
@@ -33,3 +34,7 @@
 
 You can use the "-c" flag to test whether a zip archive is properly aligned.
 
+The "-p" flag aligns any file with a ".so" extension, and which is stored
+uncompressed in the zip archive, to a 4096-byte page boundary.  This
+facilitates directly loading shared libraries from inside a zip archive.
+