Expand tests to all branches.

Also update README with testing badge.

.github/workflows/python-package.yml

@@ -1,13 +1,13 @@
 # This workflow will install Python dependencies, run tests and lint with a variety of Python versions
 # For more information see: https://help.github.com/actions/language-and-framework-guides/using-python-with-github-actions
 
-name: Test coverage
+name: Tests
 
 on:
   push:
-    branches: [ master ]
+    branches: '**'
   pull_request:
-    branches: [ master ]
+    branches: '**'
 
 jobs:
   build: