message: README: add a section for test coverage
  Summary:
  It is a hassle to figure out the exact commands to create a good
  report.
  This is a backport of libsecp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/704 | PR704]]
  Depends on D5383
  Test Plan: Read
  Reviewers: #bitcoin_abc, Fabien
  Subscribers: Fabien
diff: @@ -85,6 +85,27 @@ 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
  ------------
  