doc: instructions for generating coverage reports

Add instructions for generating code coverage reports to BUILDING.md

BUILDING.md

@@ -126,6 +126,17 @@ To run the tests:
 $ make test
 ```
 
+To run the tests and generate code coverage reports:
+
+```console
+$ ./configure --coverage
+$ make coverage
+```
+
+This will generate coverage reports for both JavaScript and C++ tests (if you
+only want to run the JavaScript tests then you do not need to run the first
+command `./configure --coverage`).
+
 To build the documentation:
 
 This will build Node.js first (if necessary) and then use it to build the docs: