Add feature flag hint to README.md, fixes #1633 (#1709)

tokenizers/README.md

@@ -42,6 +42,7 @@ use tokenizers::tokenizer::{Result, Tokenizer};
 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)?;