adds README section regarding usage in child procs (#850)

* adds README section regarding usage in child procs

code example and original request copied from @aaarichter
should close #811

* Update README.md

Co-authored-by: Qix <Qix-@users.noreply.github.com>

Co-authored-by: Qix <Qix-@users.noreply.github.com>

README.md

@@ -351,6 +351,27 @@ if (debug.enabled) {
 You can also manually toggle this property to force the debug instance to be
 enabled or disabled.
 
+## Usage in child processes
+
+Due to the way `debug` detects if the output is a TTY or not, colors are not shown in child processes when `stderr` is piped. A solution is to pass the `DEBUG_COLORS=1` environment variable to the child process.  
+For example:
+
+```javascript
+worker = fork(WORKER_WRAP_PATH, [workerPath], {
+  stdio: [
+    /* stdin: */ 0,
+    /* stdout: */ 'pipe',
+    /* stderr: */ 'pipe',
+    'ipc',
+  ],
+  env: Object.assign({}, process.env, {
+    DEBUG_COLORS: 1 // without this settings, colors won't be shown
+  }),
+});
+
+worker.stderr.pipe(process.stderr, { end: false });
+```
+
 
 ## Authors