Add note about initializing submodules needed to build successfully.

9 jobs for build-docs-submodules in 9 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #70142
docker linux amd64
build:linux:amd64

00:02:12

 
  Test
passed #70153
docker linux amd64
deckard:linux:amd64

00:00:35

passed #70152
docker linux amd64
deckard:linux:amd64:valgrind

00:01:12

passed #70147
docker linux amd64
respdiff:linux:amd64

00:03:42

passed #70143
docker linux amd64
test:linux:amd64

00:02:51

passed #70146
docker linux amd64
test:linux:amd64:valgrind

00:02:34

failed #70144
docker linux amd64
deckard:linux:amd64

00:03:08

failed #70151
docker linux amd64
deckard:linux:amd64

00:00:39

failed #70145
docker linux amd64
deckard:linux:amd64:valgrind

00:04:21