added pkgdown github action script; contributes to #16

.github/workflows/pkgdown.yaml

@@ -0,0 +1,32 @@
+on:
+  push:
+    branches: master
+  pull_request:
+    branches: master
+
+name: pkgdown
+
+jobs:
+  pkgdown:
+    runs-on: ubuntu-latest
+    env:
+      GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: r-lib/actions/setup-pandoc@v1
+
+      - uses: r-lib/actions/setup-r@v1
+        with:
+          use-public-rspm: true
+
+      - uses: r-lib/actions/setup-r-dependencies@v1
+        with:
+          extra-packages: pkgdown
+          needs: website
+
+      - name: Deploy package
+        run: |
+          git config --local user.name "$GITHUB_ACTOR"
+          git config --local user.email "$GITHUB_ACTOR@users.noreply.github.com"
+          Rscript -e 'pkgdown::deploy_to_branch(new_process = FALSE)'