Mention submodules in getting started doc (#4719)

Mention submodules in getting started doc

Author: adriaanm

docs/docs/contributing/getting-started.md

@@ -16,12 +16,15 @@ Compiling and Running
 Start by cloning the repository:
 
 ```bash
-$ git clone --recursive --single-branch https://github.com/lampepfl/dotty.git
+$ git clone --recurse-submodules --single-branch https://github.com/lampepfl/dotty.git
 $ cd dotty
 $ sbt managedSources # Needed for IDE import to succeed
 ```
+
 Pass `--single-branch` to clone only the master branch, otherwise cloning will be *much* slower (details in [issue #3236](https://github.com/lampepfl/dotty/issues/3236)).
 
+The repository uses submodules, which may need updating/initializing using `git submodule update --init`. Starting `sbt` will alert you if this is needed.
+
 Dotty provides a standard sbt build: compiling, running and starting a repl can
 all be done from within sbt: