ci: add workflow to backup issues using gh2md

See #11.

.github/workflows/issues2md.yml

@@ -0,0 +1,37 @@
+# Action to backup issues using gh2md, contributed by
+# https://github.com/0ut0fcontrol.
+# See https://github.com/mattduck/gh2md/issues/11.
+name: Issues2Markdown
+on:
+  push:
+  schedule:
+    # every day
+    - cron: "0 0 * * *"
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+      with:
+        persist-credentials: false # otherwise, the token used is the GITHUB_TOKEN, instead of your personal token
+        fetch-depth: 0 # otherwise, you will failed to push refs to dest repo
+    - name: Backup github issues to a markdown file.
+      run: |
+        pip install wheel
+        pip install --user gh2md
+        $HOME/.local/bin/gh2md $GITHUB_REPOSITORY issues.md --token ${{ secrets.GITHUB_TOKEN }}
+        git add issues.md
+    - name: Commit files
+      run: |
+        git config --local user.email "[email protected]"
+        git config --local user.name "GitHub Action"
+        git commit -m "Backup all issues into issues.md" -a
+    - name: Extract branch name
+      shell: bash
+      run: echo "##[set-output name=branch;]$(echo ${GITHUB_REF#refs/heads/})"
+      id: extract_branch
+    - name: Push changes
+      uses: ad-m/github-push-action@master
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        branch: ${{ steps.extract_branch.outputs.branch }}