docs: add submodule update step prior to 'make release' (tinygo-org#1213)

* docs: add submodule update step prior to 'make release'

Author: brandentimm

BUILDING.md

@@ -92,6 +92,10 @@ Now that we have a working static build, it's time to make a release tarball:
 
     make release
 
+If you did not clone the repository with the `--recursive` option, you will get errors until you initialize the project submodules:
+
+    git submodule update --init
+
 The release tarball is stored in build/release.tar.gz, and can be extracted with
 the following command (for example in ~/lib):