Make the workflow file friendlier to coying to other projects

We link it from the README so people will probably copy-paste this.

- Say "Tox tests" rather than "example tests"
- Explain what the ubuntu is

Author: encukou

.github/workflows/main.yml

@@ -6,19 +6,21 @@ on:
     branches:
     - master
 
-name: Tox sanity test
+name: Run Tox tests
 
 jobs:
-  test_job:
-    runs-on: ubuntu-latest
+  tox_test:
     name: Tox test
     steps:
     - uses: actions/checkout@v2
-    - name: Run an example test
+    - name: Run Tox tests
       id: test
       uses: fedora-python/tox-github-action@master
       with:
         tox_env: ${{ matrix.tox_env }}
     strategy:
       matrix:
         tox_env: [py27, py35, py36, py37, py38, py39, pypy2, pypy3]
+
+    # Use GitHub's Linux Docker host
+    runs-on: ubuntu-latest