pitft: Add documentation for pitft35r in README

The support for the PiTFT 3.5 inch resistive touchscreen is optional.
It may be enabled by adding the following line in local.conf:

    MACHINE_FEATURES += "pitft pitft35r"

Signed-off-by: Rob Woolley <rob.woolley@windriver.com>

README

@@ -242,6 +242,7 @@ the modelname should be added as a MACHINE_FEATURES in local.conf like below:
 List of currently supported models:
     - pitft22
     - pitft28r
+    - pitft35r
 
 3.O. Misc. display
 ==================