Documentation: Add Gigabyte to list of mainboards

Gigabyte was not in the list of vendors in the mainboard-specific
documentation. This made a newly added mainboard page difficult to
locate. This commit adds Gigabyte and links said mainboard in the
mainboard-specific documentation main page.

Change-Id: I8839e1c1176fbdc3dd9da616f68c58e8e1cf1b16
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/27045
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
diff --git a/Documentation/mainboard/index.md b/Documentation/mainboard/index.md
index 198211d..5d41cec 100644
--- a/Documentation/mainboard/index.md
+++ b/Documentation/mainboard/index.md
@@ -6,6 +6,10 @@
 
 - [CN81XX EVB SFF](cavium/cn8100_sff_evb.md)
 
+## Gigabyte
+
+- [GA-H61M-S2PV](gigabyte/ga-h61m-s2pv.md)
+
 ## HP
 
 - [Compaq 8200 Elite SFF](hp/compaq_8200_sff.md)