Added RAM split detection, added offline mode, added option to skip kernel update (thanks AndrewS!)

README.md

@@ -21,7 +21,10 @@ To then update your firmware, simply run the following command as root:
 rpi-update
 </pre>
 
-By default, the 224MB memory split will be used. If you'd like to use the 192MB split, then type:
+
+By default, rpi-update will attempt to determine the split you're currently using, and then use that split. If it cannot determine what split you are using, it will default to 224MB.
+
+If you'd like to explicitly select a split, simply provide the RAM split value after the command as follows:
 
 <pre>
 rpi-update 192
@@ -32,3 +35,28 @@ If you'd like to use the 128MB memory split, then the command is the same as the
 After the firmware has been sucessfully updated, you'll need to reboot to load the new firmware.
 
 This tool is experimental, and may screw up your install. If you have problems with it, post an issue to this GitHub repo and I'll see if I can help you.
+
+Expert options
+--------------
+
+There are a number of options for experts you might like to use, these are all environment variables you must set if you wish to use them.
+
+### SKIP_KERNEL
+
+#### Usage
+
+SKIP_KERNEL=1 rpi-update
+
+#### Effect
+
+Will update everything EXCEPT the kernel.img files and the kernel modules. Use with caution, some firmware updates might depend a kernel update.
+
+### ROOT_PATH/BOOT_PATH
+
+#### Usage
+
+ROOT_PATH=/media/root BOOT_PATH=/media/boot rpi-update
+
+#### Effect
+
+Allows you to perform an "offline" update, ie update firmware on an SD card you're not currently booted from. Useful for installing firmware/kernel to a non-RPI customised image. Be careful, you must specify both options or neither. Specifying only one will not work.