Add psalm GitHub action

Author: klimick

.github/workflows/php.yml

@@ -0,0 +1,39 @@
+name: PHP Composer
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Install dependencies
+      run: composer install --prefer-dist --no-progress
+
+    - name: Cache Composer dependencies
+      id: composer-cache
+      uses: actions/cache@v2
+      with:
+        path: vendor
+        key: ${{ runner.os }}-php-${{ github.sha }}
+
+  psalm:
+    runs-on: ubuntu-latest
+    needs:
+      - build
+    steps:
+      - uses: actions/checkout@v2
+      - name: Restore cached composer dependencies
+        id: composer-cache
+        uses: actions/cache@v2
+        with:
+          path: vendor
+          key: ${{ runner.os }}-php-${{ github.sha }}
+
+      - name: Run psalm validation
+        run: vendor/bin/psalm --output-format=github --shepherd