Dev guide: In Code Coverage section, add notes about CodeCov

source/developers/references/developer_guide/testing/code-coverage.rst

@@ -2,6 +2,30 @@
 Code Coverage
 #############
 
-# TODO: Explain how to get a coverage report locally
+We use a coverage tool to test on your pull request whether or not your changed
+lines of code are _covered_ by unit tests. That means, for each line, is there a
+unit test that tests out that line?
+
+Unit testing is a software testing method where “units” — the individual
+components of software—are tested. Developers write unit tests for their code to
+make sure that the code works correctly. This helps to detect and protect
+against bugs in the future.
+
+The main objective of unit testing is to isolate written code to test and
+determine if it works as intended. Unit testing is an important step in the
+development process. If done correctly, unit tests can detect early flaws in
+code and prevent future developers from breaking your feature by enforcing its
+correct behavior.
+
+In ``edx-platform``, coverage is run via `CodeCov
+<https://about.codecov.io/resource/what-is-code-coverage/>`_ and you can see the
+results of your Codecov by viewing the coverage builds:
+
+.. image:: /_images/devguide_coverage_builds.png
+
+Other repositories may use CodeCov or other coverage tools; check the builds on
+your pull request, the repo's Makefile, or the repo's README for more detail. It
+is your responsibility to make sure you provide adequete test coverage for all
+changes you make.