Document updating submodules

Author: Nemo157

README.md

@@ -56,6 +56,7 @@ docker is installed and the service is running.
 ```sh
 git clone https://github.com/rust-lang/docs.rs.git docs.rs
 cd docs.rs
+git submodule update --init
 # Configure the default settings for external services
 cp .env.sample .env
 # Create the DOCSRS_PREFIX directory

build.rs

@@ -79,7 +79,12 @@ fn main() -> Result<()> {
     write_git_version(out_dir)?;
     compile_sass(out_dir)?;
     write_known_targets(out_dir)?;
-    compile_syntax(out_dir)?;
+    compile_syntax(out_dir).context(
+        "\
+            could not compile syntax files\n\n\
+            Note: you may need to run `git submodule update --init`\
+        ",
+    )?;
     Ok(())
 }