Add multiline comment syntax. Fixes #171.

src/ch03-04-comments.md

@@ -21,6 +21,14 @@ single line, you’ll need to include `//` on each line, like this:
 // explain what’s going on.
 ```
 
+Or you can use the multiline comment syntax with `/*` and `*/`:
+
+```rust
+/* So we’re doing something complicated here, long enough that we need
+   multiple lines of comments to do it! Whew! Hopefully, this comment will
+   explain what’s going on. */
+```
+
 Comments can also be placed at the end of lines containing code:
 
 <span class="filename">Filename: src/main.rs</span>