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 [[bitcoin-core/secp256k1#704 | PR704]]

Depends on D5383

Test Plan: Read

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5387

src/secp256k1/README.md

@@ -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
 ------------