Documentation: Start administrator handbook

Let's spread the work of maintaining various of our services, but to
achieve that, we need to document what needs to be done.

Change-Id: I87021ee62d18fa464f70351ea8bad732889d55f1
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79901
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/infrastructure/index.md

@@ -5,7 +5,7 @@ This section contains documentation about our infrastructure
 ## Services
 
 * [Project services](services.md)
-
+* [Administrator's handbook](admin.md)
 
 ## Jenkins builders and builds
 * [Setting up Jenkins build machines](builders.md)