[doc] Document expected environment for dib

diskimage-builder, or at least the most common elements, have
always had an implicit requirement to be run code intended for a
linux machine on the targeted architecture. This documents that
requirement up-front so users won't attempt in vain to build
cross-architecture or use it unix-like systems like Mac OS.

Change-Id: I22be1f9c929054c8ddc3253f55fa310bee29c055

doc/source/user_guide/building_an_image.rst

@@ -4,6 +4,10 @@ Building An Image
 Now that you have diskimage-builder properly :doc:`installed <installation>`
 you can get started by building your first disk image.
 
+.. note:: diskimage-builder requires a linux machine able to mount filesystems
+          and execute code for the target CPU architecture of the image. (e.g.;
+          building an ARM image requires a linux machine that can run ARM code).
+
 VM Image
 --------