mmc: documentation of mmc non-blocking request usage and design.

Documentation about the background and the design of mmc non-blocking.
Host driver guidelines to minimize request preparation overhead.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Chris Ball <cjb@laptop.org>
diff --git a/Documentation/mmc/00-INDEX b/Documentation/mmc/00-INDEX
index 93dd7a7..a9ba672 100644
--- a/Documentation/mmc/00-INDEX
+++ b/Documentation/mmc/00-INDEX
@@ -4,3 +4,5 @@
         - info on SD and MMC device attributes
 mmc-dev-parts.txt
         - info on SD and MMC device partitions
+mmc-async-req.txt
+        - info on mmc asynchronous requests