Document MSRV effect of different versions of hashes

We use a range dependency for `bitcoin_hashes`, the different allowed
versions each have a different MSRV, which implies using each changes
the MSRV of `bip39`.

Author: tcharding

README.md

@@ -31,6 +31,11 @@ Use the `all-languages` feature to enable all languages.
 
 This crate supports Rust v1.41.1 and up and works with `no_std`.
 
+The `bitcoin_hashes` range dependency effects the MSRV as follows
+
+- `bitcoin_hashes v0.12`: MSRV v1.41.1
+- `bitcoin_hashes v0.13`: MSRV v1.48.0
+
 When using older version of Rust, you might have to pin the version of the
 `bitcoin_hashes` crate used as such:
 ```