Merge #84: README: fix MSRV

35ec014 README: fix MSRV (Jonas Nick)

Pull request description:

  MSRV was increased in a previous commit, but the README was not updated.

  Fixes #83.

ACKs for top commit:
  apoelstra:
    ACK 35ec014 successfully ran local tests

Tree-SHA512: d96092cd8d87f6a0aa3978a25d59a41c09fd9c0ba5f8795d1dc074ec83bc7d639fad1121316cd4489dc80f9b9ad811d154add05c7782535e1f31cf6d4e09c14b

Author: apoelstra

README.md

@@ -20,5 +20,5 @@ Contributions to this library are welcome. A few guidelines:
 - Any breaking changes must have an accompanied entry in CHANGELOG.md
 - No new dependencies, please.
 - No crypto should be implemented in Rust, with the possible exception of hash functions. Cryptographic contributions should be directed upstream to libsecp256k1.
-- This library should always compile with any combination of features on **Rust 1.41.1**.
+- This library should always compile with any combination of features on **Rust 1.56.1**.