Add docs for source map output

README.md

@@ -51,6 +51,16 @@ and a `text-lcov` coverage report.
 nyc --reporter=lcov --reporter=text-lcov npm test
 ```
 
+## Stack Traces
+
+When `source-map` handling enabled, then the instrumented source files will
+include inline source maps for the instrumenter transform. When combined with
+a supporting version of [source-map-support](https://github.com/evanw/node-source-map-support/pull/118)
+(currently published under `@kpdecker/source-map-support` while PR pending),
+stack traces for instrumented code should reflect the original lines.
+
+This is enabled by default, but may be disabled by `nyc --source-map=false`.
+
 ## Support for custom require hooks (babel, webpack, etc.)
 
 nyc supports custom require hooks like