Document the 'aapt dump strings' command

Update the usage page for aapt to mention 'strings' as a valid
subcommand to 'dump'.

Change-Id: Iadb2b8ce36951ff36ffbf6b5ea349ba1e2b12582
diff --git a/tools/aapt/Main.cpp b/tools/aapt/Main.cpp
index f398de0..74ffeb0 100644
--- a/tools/aapt/Main.cpp
+++ b/tools/aapt/Main.cpp
@@ -48,6 +48,7 @@
         "   List contents of Zip-compatible archive.\n\n", gProgName);
     fprintf(stderr,
         " %s d[ump] [--values] WHAT file.{apk} [asset [asset ...]]\n"
+        "   strings          Print the contents of the resource table string pool in the APK.\n"
         "   badging          Print the label and icon for the app declared in APK.\n"
         "   permissions      Print the permissions from the APK.\n"
         "   resources        Print the resource table from the APK.\n"