bbb: add script notes for creating flasher/standalone image

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>

readme.md

 Scripts to support customized image generation for many arm systems
 
+BeagleBoard branch:
+------------
+
+    git clone git://github.com/beagleboard/image-builder.git
+
+Images:
+
+    ./beagleboard.org_image.sh
+    http://beagleboard.org/source
+
+Flasher:
+    sudo ./setup_sdcard.sh --img BBB-eMMC-flasher-debian-7.2-2013-11-21 --uboot bone --beagleboard.org-production --bbb-flasher
+    xz -z -7 -v BBB-debian-7.2-2013-11-21-2gb.img
+
+4GB, microSD:
+    sudo ./setup_sdcard.sh --img-4gb BBB-debian-7.2-2013-11-26 --uboot bone --beagleboard.org-production
+    xz -z -7 -v BBB-debian-7.2-2013-11-21-4gb.img
+
 Master branch:
 ------------