Add details on how to view coverage locally

Unfortunately for now, Coveralls reports don't include source due
to #84, but this is a good workaround.

CONTRIBUTING.md

@@ -90,6 +90,18 @@ recommended to put them in a subdirectory called `playground/` since
 this will be automatically ignored by git and the build process,
 avoiding any undesirable complications.
 
+Test coverage
+~~~~~~~~~~~~~
+
+To view test coverage reports, first ensure that
+[`Devel::Cover`](https://metacpan.org/dist/Devel-Cover) is installed.
+Then type `make coverage`.  The last lines of the output should
+include something like:
+
+    HTML output written to /home/user/path/to/stow/cover_db/coverage.html
+
+which you can open in a web browser to view the report.
+
 Translating Stow
 ----------------
 

Makefile.am

@@ -87,6 +87,10 @@ check-TESTS:
 	dir=$(TESTS_DIR); \
 	$(TESTS_ENVIRONMENT) -MTest::Harness -e 'runtests(@ARGV)' "$${dir#./}"/*.t
 
+coverage:
+	PERL5OPT=-MDevel::Cover $(MAKE) check-TESTS
+	cover
+
 $(TESTS_OUT):
 	mkdir -p $@