Documentation: Add mainboard section to index

Change-Id: Ib00bd6cdd13f164ca7eb07c9092e8d8fe17d23b1
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/26266
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/index.md

@@ -19,3 +19,4 @@ Contents:
 * [GPIO toggling in ACPI AML](acpi/gpio.md)
 * [Native Graphics Initialization with libgfxinit](gfx/libgfxinit.md)
 * [Sandy Bridge Raminit](Intel/NativeRaminit/Sandybridge.md)
+* [Mainboard-specific documentation](mainboard/index.md)

Documentation/mainboard/index.md

@@ -0,0 +1,7 @@
+# Mainboard-specific documentation
+
+This section contains documentation about coreboot on specific mainboards.
+
+## SiFive
+
+- [SiFive HiFive Unleashed](sifive/hifive-unleashed.md)