docs(chain): use doc_cfg feature

Author: evanlinjin

crates/chain/src/lib.rs

@@ -17,6 +17,12 @@
 //!
 //! [Bitcoin Dev Kit]: https://bitcoindevkit.org/
 
+// only enables the `doc_cfg` feature when the `docsrs` configuration attribute is defined
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(
+    docsrs,
+    doc(html_logo_url = "https://github.com/bitcoindevkit/bdk/raw/master/static/bdk.png")
+)]
 #![no_std]
 #![warn(missing_docs)]