readme: Use subheader markdown

Use header level two (`##`) for subheader. Add spacing below headings
while we are at it.

Author: tcharding

README.md

@@ -1,4 +1,5 @@
 # Bech32 Rust
+
 [![Docs.rs badge](https://docs.rs/bech32/badge.svg)](https://docs.rs/bech32/)
 [![Continuous Integration](https://github.com/rust-bitcoin/rust-bech32/workflows/Continuous%20Integration/badge.svg)](https://github.com/rust-bitcoin/rust-bech32/actions?query=workflow%3A%22Continuous+Integration%22)
 
@@ -9,6 +10,8 @@ You can find some usage examples in the [documentation](https://docs.rs/bech32/)
 
 Bitcoin-specific address encoding is handled by the `bitcoin-bech32` crate.
 
-# MSRV
+
+## MSRV
+
 The minimum supported Rust version with the standard library is **1.41.1**.