doc: indentation rules for element

This change defines indentation rules for the elements. It enforces
the use of 4 spaces which is already the devstack standard.

Change-Id: I072cf8bf6748d0c910fecffdf2282bcc4656d038

README.md

@@ -358,6 +358,13 @@ Ramdisk elements support the following files in their element directories:
 
 * udev.d : udev rules files that will be copied into the ramdisk.
 
+### Whitespace Rules
+
+- lines should not include trailing whitespace
+- there should be no hard tabs in the file
+- indents are 4 spaces, and all indentation should be some multiple of
+  them
+
 Global image-build variables
 ----------------------------