chore: add update to clientbuilder doc (#280)

src/lib.rs

@@ -164,6 +164,15 @@ mod session;
 /// [`rustls`](ClientBuilder::rustls) (depending on your preference) to start the builder. If you
 /// want to supply your own connector, use [`new`](ClientBuilder::new).
 ///
+/// Note: [`geckodriver`](https://github.com/mozilla/geckodriver) does not support multiple
+/// simultaneous instances. When using geckodriver, ensure you:
+///
+/// - Run only one webdriver instance at a time
+/// - Explicitly close the webdriver session after use, even if an error occurs
+///
+/// To run multiple webdriver instances use
+/// [chromedriver](https://developer.chrome.com/docs/chromedriver/downloads).
+///
 /// To connect to the WebDriver instance, call [`connect`](ClientBuilder::connect).
 ///
 /// [builder]: https://rust-lang.github.io/api-guidelines/type-safety.html#c-builder