Add gh workflow

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

.github/workflows/static-analysis.yml

+name: Static analysis
+
+on: [push]
+
+jobs:
+  static-psalm-analysis:
+      name: Static analysis with Psalm
+      runs-on: ubuntu-latest
+      steps:
+          - name: Checkout
+            uses: actions/checkout@master
+          - name: Set up php
+            uses: shivammathur/setup-php@master
+            with:
+                php-version: 7.4
+                coverage: none
+          - name: Install dependencies
+            run: composer i
+          - name: Run coding standards check
+            run: composer run psalm