Remove stale MSRV docs

We have stale docs referring to the old MSRV. We do not need MSRV docs
in `CONTRIBUTING` because we have them in the README already.

Fix stale docs by doing:
- Remove the MSRV docs from readme in favour of `CONTRIBUTING.md`.
- Add a sentence to the redame pointing readers towards `CONTRIBUTING.md`.

Author: tcharding

CONTRIBUTING.md

@@ -183,12 +183,6 @@ and [how it is planned to coordinate it with crate refactoring](https://github.c
 For the new code it is recommended to follow style of the existing codebase and
 avoid any end-line space characters.
 
-### MSRV
-
-The Minimal Supported Rust Version (MSRV) is 1.29; it is enforced by our CI.
-Later we plan to increase MSRV to support Rust 2018 and you are welcome to check
-the [tracking issue](https://github.com/rust-bitcoin/rust-bitcoin/issues/510).
-
 ### Naming conventions
 
 Naming of data structures/enums and their fields/variants must follow names used

README.md

@@ -71,6 +71,8 @@ please join us in
 [#bitcoin-rust](https://web.libera.chat/?channel=#bitcoin-rust) on
 [libera.chat](https://libera.chat).
 
+For more information please see `./CONTRIBUTING.md`.
+
 ## Minimum Supported Rust Version (MSRV)
 
 This library should always compile with any combination of features (minus