Update manifest and README

Author: jakobnissen

Manifest.toml

@@ -1,6 +1,4 @@
-## Release 0.3
-
-__Breaking changes__
+## Release 0.3.1
 
 __New features__
 * New function: `base(::Option{T})`. Converts an `Option{T}` into a `Union{Some{T}, Nothing}`.

NEWS.md

@@ -2,7 +2,6 @@
 
 ![CI](https://github.com/jakobnissen/ErrorTypes.jl/workflows/CI/badge.svg)
 [![Codecov](https://codecov.io/gh/jakobnissen/ErrorTypes.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/jakobnissen/ErrorTypes.jl)
-[![](https://img.shields.io/badge/docs-stable-blue.svg)](https://jakobnissen.github.io/ErrorTypes.jl/stable)
 [![](https://img.shields.io/badge/docs-dev-blue.svg)](https://jakobnissen.github.io/ErrorTypes.jl/dev)
 
 _Rust-like safe error handling in Julia_