Create cffconvert.yml (#4534)

.github/workflows/cffconvert.yml

+name: cffconvert
+
+on:
+  push:
+    paths:
+      - CITATION.cff
+
+jobs:
+  validate:
+    name: "validate"
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out a copy of the repository
+        uses: actions/checkout@v2
+
+      - name: Check whether the citation metadata from CITATION.cff is valid
+        uses: citation-file-format/cffconvert-github-action@2.0.0
+        with:
+          args: "--validate"