doc: adds link to nightly code coverage report

Adds a link to the nightly test code coverage report page
to the writing-tests.md documentation.

doc/guides/writing-tests.md

@@ -424,6 +424,9 @@ will depend on what is being tested if this is required or not.
 To generate a test coverage report, see the
 [Test Coverage section of the Building guide][].
 
+Nightly coverage reports for the Node.js master branch are available at
+https://coverage.nodejs.org/.
+
 [ASCII]: http://man7.org/linux/man-pages/man7/ascii.7.html
 [Google Test]: https://github.com/google/googletest
 [`common` module]: https://github.com/nodejs/node/blob/master/test/common/README.md