Add temporary warning about edition change

This warns contributors about possible rebases. Changing CONTRIBUTING
should ensure that GitHub will display special warning. This will be
removed after migration is done.

Author: Kixunil

CONTRIBUTING.md

@@ -1,5 +1,11 @@
 # Contributing to rust-bitcoin
 
+**Heads up about upcoming edition change**
+
+We're currently preparing to bump MSRV and **change the edition to 2018**.
+To minimize the churn we recommend you to submit your local WIP changes ASAP.
+There will be a lot of rebasing after the edition change.
+
 :+1::tada: First off, thanks for taking the time to contribute! :tada::+1:
 
 The following is a set of guidelines for contributing to Rust Bitcoin

README.md

@@ -18,6 +18,11 @@
   </p>
 </div>
 
+**Heads up for contributors: upcoming edition change**
+
+We're currently preparing to bump MSRV and **change the edition to 2018**.
+To minimize the churn we recommend you to submit your local WIP changes ASAP.
+There will be a lot of rebasing after the edition change.
 
 [Documentation](https://docs.rs/bitcoin/)