Add CodeCov badge to README and create .codecov.yml

Co-authored-by: leeoades <2321091+leeoades@users.noreply.github.com>

Author: Copilot

.codecov.yml

@@ -0,0 +1,19 @@
+coverage:
+  status:
+    project:
+      default:
+        target: 80%
+        threshold: 2%
+    patch:
+      default:
+        target: 80%
+        threshold: 5%
+
+comment:
+  layout: "reach,diff,flags,tree"
+  behavior: default
+  require_changes: true
+
+ignore:
+  - "samples/**"
+  - "test/FunctionalStateMachine.Benchmarks/**"

README.md

@@ -3,6 +3,7 @@
 [![NuGet](https://img.shields.io/nuget/v/FunctionalStateMachine.Core.svg)](https://www.nuget.org/packages/FunctionalStateMachine.Core/)
 [![NuGet Downloads](https://img.shields.io/nuget/dt/FunctionalStateMachine.Core.svg)](https://www.nuget.org/packages/FunctionalStateMachine.Core/)
 [![CI](https://github.com/leeoades/FunctionalStateMachine/actions/workflows/ci.yml/badge.svg)](https://github.com/leeoades/FunctionalStateMachine/actions/workflows/ci.yml)
+[![codecov](https://codecov.io/gh/leeoades/FunctionalStateMachine/graph/badge.svg)](https://codecov.io/gh/leeoades/FunctionalStateMachine)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
 **A persistence-friendly state machine for .NET that returns commands instead of executing side effects.**