Merge #11680: [docs] Add instructions for lcov report generation

5ff01c236 [docs] Add instructions for lcov coverage report generation (James O'Beirne)

Pull request description:

  After rediscovering the `lcov` report generation recipe one too many times, it seemed prudent to write some doc.

Tree-SHA512: 20e1b5f51ecd39e14bd67986a2c1578fb7da03a50625366eaca35b201db66aef99cd4a5456df3aaca5d2d66b18ed7d2e8eb8f3bd9c7aaf9af48164d9bac38931

doc/developer-notes.md

@@ -182,6 +182,22 @@ $ valgrind --suppressions=contrib/valgrind.supp --leak-check=full \
 $ valgrind -v --leak-check=full src/bitcoind -printtoconsole
 ```
 
+**compiling for test coverage**
+
+LCOV can be used to generate a test coverage report based upon `make check`
+execution. LCOV must be installed on your system (e.g. the `lcov` package
+on Debian/Ubuntu).
+
+To enable LCOV report generation during test runs:
+
+```shell
+./configure --enable-lcov
+make
+make cov
+
+# A coverage report will now be accessible at `./test_bitcoin.coverage/index.html`.
+```
+
 Locking/mutex usage notes
 -------------------------