github: attempt to build release using rust-build/rust-build.action (#73)

I just copied from the examples on
https://github.com/rust-build/rust-build.action. Let's see if it
works.

.github/workflows/release.yml

@@ -0,0 +1,29 @@
+# .github/workflows/release.yml
+
+on:
+  release:
+    types: [created]
+
+jobs:
+  release:
+    name: release ${{ matrix.target }}
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+        - target: x86_64-pc-windows-gnu
+          archive: zip
+        - target: x86_64-unknown-linux-musl
+          archive: tar.gz tar.xz
+        - target: x86_64-apple-darwin
+          archive: zip
+    steps:
+    - uses: actions/checkout@master
+    - name: Compile and release
+      uses: rust-build/rust-build.action@latest
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        RUSTTARGET: ${{ matrix.target }}
+        EXTRA_FILES: "README.md LICENSE"
+        ARCHIVE_TYPES: ${{ matrix.archive }}