Allow image building to be skipped for stages (#137)

README.md

@@ -228,14 +228,14 @@ replace with your own contents in the same format.
 If you're working on a specific stage the recommended development process is as
 follows:
 
- * Remove the EXPORT_IMAGE and EXPORT_NOOBS files until you're ready to actually
-   generate an image
+ * Add a file called SKIP_IMAGES into the directories containing EXPORT_* files
+   (currently stage2, stage4 and stage5)
  * Add SKIP files to the stages you don't want to build. For example, if you're
    basing your image on the lite image you would add these to stages 3, 4 and 5.
  * Run build.sh to build all stages
  * Add SKIP files to the earlier successfully built stages
  * Modify the last stage
  * Rebuild just the last stage using ```sudo CLEAN=1 ./build.sh```
- * Once you're happy with the image you can add the EXPORT files back in and
+ * Once you're happy with the image you can remove the SKIP_IMAGES files and
    export your image to test