Add coverage workflow

.github/workflows/main.yml

@@ -0,0 +1,21 @@
+on:
+  push:
+    branches:
+      - main
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-go@v3
+      
+      - name: generate test coverage
+        run: go test ./... -coverprofile=./cover.out -covermode=atomic -coverpkg=./...
+
+      - name: check test coverage
+        uses: vladopajic/go-test-coverage@v2
+        with:
+          profile: cover.out
+          local-prefix: github.com/gabehf/go-elo
+          git-token: ${{ github.ref_name == 'main' && secrets.GITHUB_TOKEN || '' }}
+          git-branch: badges 

README.md

@@ -1,5 +1,7 @@
 # go-elo: A flexible elo calculator for Go
 
+![coverage](https://raw.githubusercontent.com/gabehf/go-elo/badges/.badges/main/coverage.svg)
+
 This package allows you to add flexible elo/skill tracking to your application.
 
 Go-elo supports unscored (win/loss) and scored matches, with options to tailor the elo curve to your liking.