document dist.vendor in config.example.toml

Signed-off-by: onur-ozkan <work@onurozkan.dev>

config.example.toml

@@ -942,3 +942,6 @@
 # Copy the linker, DLLs, and various libraries from MinGW into the Rust toolchain.
 # Only applies when the host or target is pc-windows-gnu.
 #include-mingw-linker = true
+
+# Whether to vendor dependencies for the dist tarball.
+#vendor = if "is a tarball source" || "is a git repository" { true } else { false }