Add GitHub Actions workflow to build the JOSS paper

.github/workflows/paper.yml

+name: Paper
+
+on:
+  push:
+    branches:
+      - paper
+
+jobs:
+  paper:
+    runs-on: ubuntu-latest
+    name: Paper Draft
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Build draft PDF
+        uses: openjournals/openjournals-draft-action@master
+        with:
+          journal: joss
+          paper-path: paper/paper.md
+      - name: Upload
+        uses: actions/upload-artifact@v1
+        with:
+          name: paper.pdf
+          path: paper/paper.pdf