Add BC checker (#16)

.github/workflows/bc_check.yaml

@@ -0,0 +1,24 @@
+name: BC Check
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+jobs:
+  bc_check:
+    name: Backwards Compatibility Check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: 7.4
+      - name: Install Checker
+        run: composer require --dev roave/backward-compatibility-check
+      - name: Check for BC breaks
+        run: vendor/bin/roave-backward-compatibility-check