Mention submodules in the build instructions.
Without fetching the submodules, compilation and installation is successful,
but an error occurs when starting the demo cluster.
@@ -46,6 +46,9 @@ Follow [appropriate linux steps](README.Linux.md) for getting your system ready
### Build the database
```
+# Initialize and update submodules in the repository
+git submodule update --init
+
# Configure build environment to install at /usr/local/gpdb
./configure --with-perl --with-python --with-libxml --with-gssapi --prefix=/usr/local/gpdb
