Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!
coreboot runs on a bunch of ARM platforms. See src/soc
The first fully supported ARM platform in coreboot is the Samsung XE303CE Chromebook ("snow"). It features an Exynos5250 SoC with a dual-core Cortex-A15 (ARMv7) processor. Exynos5420 support has also been added. More info about the Exynos5 SoCs and Coreboot is available here.
TI Beaglebone Black with the TI AM335x (ARM Cortex-A8) support is a work-in-progress. As of writing, basic infrastructure has been added, though full support is incomplete.
ARM SOCs with PCIe are now on the market for tablets, netbooks and servers. These systems can take advantage of coreboot's strength in properly configuring PCI, SAS, SATA and SCSI devices; fast boot times; and payload support.