Add documentation about gclient getdep.

Bug: 1475770
Change-Id: I933a912599b09f0bca4b61656c66692183dbab67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4827291
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1190862}

docs/git_submodules.md

@@ -177,6 +177,18 @@ following command:
 git restore --staging <affected path>
 ```
 
+### How do I see what revision is pinned?
+
+`gclient getdep` will return whatever commit is pinned for the deps in `DEPS`
+(unstaged, staged, or committed). If the repo is using git submodules only
+(and has no git deps in `DEPS`) it will return the whatever pinned commit is
+staged or committed.
+
+```
+gclient getdep -r <path>
+```
+
+
 If you want to keep your gitlink, then run `git add <affected path>`.
 
 ### How can I provide feedback?