[fix](manual inst): Add Git submodule instruction

In light of `gtest` and `gmock` now being submodules.

Signed-off-by: Dom Rodriguez <shymega@shymega.org.uk>

Building-on-Linux.md

@@ -38,6 +38,7 @@ git clone https://github.com/debauchee/barrier.git
 # you can get release tarballs instead
 # if you want to build from a specific tag/release
 cd barrier
+git submodule update --init --recursive
 ./clean_build.sh
 cd build
 sudo make install # install to /usr/local/