update docs to note new develop branch

Author: eeeebbbbrrrr

README.md

@@ -209,10 +209,13 @@ will be built into the `cargo-pgx` subcommand and make use of https://github.com
 We are most definitely open to contributions of any kind.  Bug Reports, Feature Requests, Documentation,
 and even [sponsorships](https://github.com/sponsors/eeeebbbbrrrr).
 
+If you'd like to contribute code via a Pull Request, please make it against our `develop` branch.  The `master` branch is meant to represent what is currently available on crates.io.
+
 Providing wrappers for Postgres' internals is not a straightforward task, and completely wrapping it is going
 to take quite a bit of time.  `pgx` is generally ready for use now, and it will continue to be developed as
 time goes on.  Your feedback about what you'd like to be able to do with `pgx` is greatly appreciated.
 
+
 ## License
 
 ```