control: README note on limited JSON support

Add a note to the README that the fan control application's JSON
support is limited to only setting fans within a zone to the configured
`full_speed`.

Change-Id: Ifd4e4515847c8766ad224ebaf27853ca4d0518ed
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/README.md b/README.md
index 4afe741..74c75d2 100755
--- a/README.md
+++ b/README.md
@@ -25,5 +25,11 @@
 * Fan presence detection(presence)
     * The error logging feature is only available via the JSON configuration.
 * Fan monitoring(monitor)
+* Fan control(control) - limited functionality
+    * Currently only supports setting fans to the configured `full_speed`  
+      (If you require more than just setting fans to the configured
+       `full_speed`, it is recommended to continue using YAML based
+       configurations until the final work can be completed to enable
+       full fan control JSON support.)
 
 To clean the repository run `./bootstrap.sh clean`.