Add tip about x flag.

Fixes rust-lang#326

src/lib.rs

@@ -161,6 +161,10 @@ assert_eq!(after, "03/14/2012, 01/01/2013 and 07/05/2014");
 # }
 ```
 
+If you wish to match against whitespace in this mode, you can still use `\s`,
+`\n`, `\t`, etc. For escaping a single space character, you can use its hex
+character code `\x20` or temporarily disable the `x` flag, e.g., `(?-x: )`.
+
 # Example: match multiple regular expressions simultaneously
 
 This demonstrates how to use a `RegexSet` to match multiple (possibly