Fixing the README. (#1714)

tokenizers/src/lib.rs

@@ -30,6 +30,7 @@
 //! fn main() -> Result<()> {
 //!     # #[cfg(feature = "http")]
 //!     # {
+//!     // needs http feature enabled
 //!     let tokenizer = Tokenizer::from_pretrained("bert-base-cased", None)?;
 //!
 //!     let encoding = tokenizer.encode("Hey there!", false)?;