Add workflow to create tarball for Linux

.github/workflows/package-linux-tarball.yml

@@ -0,0 +1,25 @@
+name: package-linux-tarball
+
+on:
+  release:
+    types: [created]
+
+jobs:
+  package:
+    runs-on: ubuntu-20.04
+
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        submodules: true
+
+    - name: Run tarball script
+      run: |
+        ./packaging/package-linux-tarball.sh .
+        mv mrtrix.tar.gz mrtrix3-$(git describe --tags --abbrev=0)-linux.tar.gz
+
+    - name: Upload package to GitHub Release
+      uses: AButler/upload-release-assets@v2.0
+      with:
+        files: '*.tar.gz'
+        repo-token: ${{ secrets.GITHUB_TOKEN }}