README: add sponsors section as promised

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

.github/workflows/sponsors.yml

@@ -0,0 +1,36 @@
+name: Generate Sponsors README
+on:
+  push:
+    branches:
+      - main
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Generate Sponsors (bronze) 💖
+        uses: JamesIves/github-sponsors-readme-action@1.0.5
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          file: 'README.md'
+          maximum: 5
+          marker: 'bronze'
+
+      - name: Generate Sponsors (silver) 💖
+        uses: JamesIves/github-sponsors-readme-action@1.0.5
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          file: 'README.md'
+          minimum: 25
+          maximum: 50
+          marker: 'silver'
+
+      - name: Generate Sponsors (gold) 💖
+        uses: JamesIves/github-sponsors-readme-action@1.0.5
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          file: 'README.md'
+          minimum: 100
+          marker: 'gold'