Add code coverage instructions to CONTRIBUTING.md

Author: hildjj

CONTRIBUTING.md

@@ -16,6 +16,7 @@ very appreciated.
 - [Submitting Pull Requests](#submitting-pull-requests)
 - [Running Tests](#running-tests)
 - [Writing New Tests](#writing-new-tests)
+- [Checking code coverage](#checking-code-coverage)
 - [Updating the `lib/mappings.wasm` WebAssembly Module](#updating-the-libmappingswasm-webassembly-module)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
@@ -113,6 +114,29 @@ can use as well:
 var util = require('./util');
 ```
 
+## Checking code coverage
+
+It's fun to find ways to test lines of code that aren't visited by
+the tests yet.
+
+```shell
+$ npm run coverage
+$ open coverage/index.html
+```
+
+This will allow you to browse to red sections of the code that need
+more attention.  Even more cool, however, is to run:
+
+```shell
+$ npm run dev
+```
+
+(On some operating systems, this may pop up a request for node to be able to open a socket.  Click "Allow" or the equivalent.)
+
+This will run the coverage tools, and monitor all of the files in the
+project, re-running the coverage tools and refreshing the browser when
+any files change.  There will be a small web server running on port 4103 to enable this.  Control-C to stop.
+
 ## Updating the `lib/mappings.wasm` WebAssembly Module
 
 Ensure that you have the Rust toolchain installed: