Merge #961: Add temporary warning about edition change

bd36d84 Add temporary warning about edition change (Martin Habovstiak)

Pull request description:

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

  See also #945

ACKs for top commit:
  sanket1729:
    ACK bd36d84
  apoelstra:
    ACK bd36d84

Tree-SHA512: 8fd0c9a0803933c3996340ceb46f6d72ec1896936267c5c14ac1a7c48499a24babad1e096fa3a903fb4aedff6b915fd0ba92dd53f8cc3fd1fa0dbccb9bf7732b

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/)