Upgrade rust/crates/structopt to 0.3.18

Test: make
Change-Id: Ib5d515b0f5e9ac7fdd0f2f8e2f00ecf54dbf63ca
diff --git a/examples/README.md b/examples/README.md
index f0db20b..b485393 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -69,6 +69,10 @@
 
 How `#[structopt(rename_all)]` works.
 
+### [Required If](required_if.rs)
+
+How to use `#[structopt(required_if)]`.
+
 ### [Skip](skip.rs)
 
 How to use `#[structopt(skip)]`.