feat(build-config-url): add link to the build configuration segment

Added an url to the "Add Build Configuration" section under tf-a
documentation to provide more insight to the patch contributor on
appending their platform specific build configurations.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I05c9285bd7905dff8261dad5678cb386f083ff85
1 file changed