Add a "coverage" task to generate a coverage report

Add a one-liner about "invoke" in docs.

docs/contribute.rst

@@ -75,6 +75,9 @@ via::
 
     invoke tests
 
+(For more on Invoke, see ``invoke -l`` to list tasks, or
+https://pyinvoke.org for documentation.)
+
 In addition to running the test suite, it is important to also ensure that any
 lines you changed conform to code style guidelines. You can check that via::
 

tasks.py

@@ -44,6 +44,13 @@ def tests(c):
     c.run(f"{VENV_BIN}/pytest", pty=PTY)
 
 
+@task
+def coverage(c):
+    """Generate code coverage of running the test suite."""
+    c.run(f"{VENV_BIN}/pytest --cov=pelican", pty=PTY)
+    c.run(f"{VENV_BIN}/coverage html", pty=PTY)
+
+
 @task
 def black(c, check=False, diff=False):
     """Run Black auto-formatter, optionally with --check or --diff"""