docs(readme): clarify pinned deps until upstream releases

Author: reez

README.md

@@ -59,6 +59,7 @@ If you have the Rust toolchain installed, the native library will be automatical
 As a user of the package, you don't need to worry about building the native library or bindings yourself.
 Only if you want to contribute to the bindings or modify the native code yourself, you can follow the instructions in [development](#development) below.
 The first build can take several minutes depending on your machine and network (subsequent builds are faster).
+We currently pin git dependencies to specific commits for reproducibility (including `bdk-ffi` until the next release is cut); the plan is to move to release tags as upstream ships them.
 
 ## Development