Add coverage badge

Author: ToucheSir

README.md

@@ -3,6 +3,7 @@
 [![][docs-stable-img]][docs-stable-url]
 [![][docs-dev-img]][docs-dev-url]
 [![][action-img]][action-url]
+[![][coverage-img]][coverage-url] 
 
 [docs-stable-img]: https://img.shields.io/badge/docs-stable-blue.svg
 [docs-stable-url]: https://fluxml.ai/Optimisers.jl/stable/
@@ -13,6 +14,9 @@
 [action-img]: https://github.com/FluxML/Optimisers.jl/workflows/CI/badge.svg
 [action-url]: https://github.com/FluxML/Optimisers.jl/actions
 
+[coverage-img]: https://codecov.io/gh/FluxML/Optimisers.jl/branch/master/graph/badge.svg
+[coverage-url]: https://codecov.io/gh/FluxML/Optimisers.jl
+
 Optimisers.jl defines many standard optimisers and utilities for learning loops.
 
 The API for defining an optimiser, and using it is simple.