Merge dashpay#704: README: add a section for test coverage

acb7f97 README: add a section for test coverage (Marko Bencun)

Pull request description:

  It is a hassle to figure out the exact commands to create a good
  report.

ACKs for top commit:
  real-or-random:
    ACK acb7f97
  sipa:
    ACK acb7f97

Tree-SHA512: d39f3e0b289229b2ce085406f6d716fdd54038df9ee5273a18a05140d1eddd4149149e881cc7a13f2126347217b9c56a0c12adf558c49879c5f556695242afc6

README.md

@@ -77,6 +77,27 @@ With valgrind, you might need to increase the max stack size:
 
     $ valgrind --max-stackframe=2500000 ./exhaustive_tests
 
+Test coverage
+-----------
+
+This library aims to have full coverage of the reachable lines and branches.
+
+To create a test coverage report, configure with `--enable-coverage` (use of GCC is necessary):
+
+    $ ./configure --enable-coverage
+
+Run the tests:
+
+    $ make check
+
+To create a report, `gcovr` is recommended, as it includes branch coverage reporting:
+
+    $ gcovr --exclude 'src/bench*' --print-summary
+
+To create a HTML report with coloured and annotated source code:
+
+    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage.html
+
 Reporting a vulnerability
 ------------