bench: update README instructions

Author: fitzgen

bench/README.md

@@ -1,15 +1,27 @@
 # Benchmarking
 
-This directory contains helpers for benchmarking (1) parsing, and (2) generating
-source maps.
+This directory contains helpers for benchmarking the `mozilla/source-map`
+library.
 
 Ensure that you have built the library, as these benchmarks rely on
 `dist/source-map.js`. See the main README.md for detais on building.
 
 ## Running Within a Browser
 
+Run a local webserver from the root of the repository:
+
+```
+$ cd source-map/
+$ python -m SimpleHTTPServer
+```
+
+Open
+[http://localhost:8000/bench/bench.html](http://localhost:8000/bench/bench.html)
+in your browser.
+
 Open `bench.html` in a browser and click on the appropriate button.
 
 ## Running with a JS Shell
 
-Run `$JS_SHELL bench-shell-bindings.js`.
+Run `$JS_SHELL bench-shell-bindings.js`, where `$JS_SHELL` is one of `node` or
+SpiderMonkey's `js`.