Add offline usage example to README

Issue: #34

Author: eriwen

README.md

@@ -5,6 +5,8 @@ stacktrace-gps - Turn partial code location into precise code location
 This library accepts a code location (in the form of a [StackFrame](https://github.com/stacktracejs/stackframe)) and
 returns a new StackFrame with a more accurate location (using [source maps](http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/)) and guessed function names.
 
+This is primarily a browser-centric library, but can be used with node.js. See the [Offline Usage section](#offline-usage) below.
+
 ## Usage
 ```js
 var stackframe = new StackFrame({fileName: 'http://localhost:3000/file.min.js', lineNumber: 1, columnNumber: 3284});
@@ -17,15 +19,34 @@ var gps = new StackTraceGPS();
 
 // Pinpoint actual function name and source-mapped location
 gps.pinpoint(stackframe).then(callback, errback);
-=> Promise(StackFrame({functionName: 'fun', fileName: 'file.js', lineNumber: 203, columnNumber: 9}), Error)
+//===> Promise(StackFrame({functionName: 'fun', fileName: 'file.js', lineNumber: 203, columnNumber: 9}), Error)
 
 // Better location/name information from source maps
 gps.getMappedLocation(stackframe).then(callback, errback);
-=> Promise(StackFrame({fileName: 'file.js', lineNumber: 203, columnNumber: 9}), Error)
+//===> Promise(StackFrame({fileName: 'file.js', lineNumber: 203, columnNumber: 9}), Error)
 
 // Get function name from location information
 gps.findFunctionName(stackframe).then(callback, errback);
-=> Promise(StackFrame({functionName: 'fun', fileName: 'http://localhost:3000/file.min.js', lineNumber: 1, columnNumber: 3284}), Error)
+//===> Promise(StackFrame({functionName: 'fun', fileName: 'http://localhost:3000/file.min.js', lineNumber: 1, columnNumber: 3284}), Error)
+```
+
+### Offline Usage
+With a bit of preparation, you can use this library offline in any environment. Any encountered fileNames not in the cache return resolved
+Promises with the original StackFrame. StackTraceGPS will make a best effort to provide as good of response with what is given and will
+fallback to the original StackFrame if nothing better could be found.
+
+```js
+var stack = ErrorStackParser.parse(new Error('boom'));
+console.assert(stack[0] == new StackFrame({fileName: 'http://localhost:9999/file.min.js', lineNumber: 1, columnNumber: 32}));
+
+var sourceCache = {'http://localhost:9999/file.min.js': 'var foo=function(){};function bar(){}var baz=eval("XXX");\n//# sourceMappingURL=file.js.map'};
+var sourceMap = '{"version":3,"sources":["./file.js"],"sourceRoot":"http://localhost:4000/","names":["foo","bar","baz","eval"],"mappings":"AAAA,GAAIA,KAAM,YACV,SAASC,QACT,GAAIC,KAAMC,KAAK","file":"file.min.js"}';
+var sourceMapConsumerCache = {'http://localhost:4000/file.js.map': new SourceMap.SourceMapConsumer(sourceMap)};
+
+var gps = new StackTraceGPS({offline: true, sourceCache: sourceCache, sourceMapConsumerCache: sourceMapConsumerCache});
+gps.pinpoint(stack[0]).then(function(betterStackFrame) {
+    console.assert(betterStackFrame === new StackFrame({functionName: 'bar', fileName: 'http://localhost:9999/file.js', lineNumber: 2, columnNumber: 9}));
+});
 ```
 
 ## Installation
@@ -39,7 +60,8 @@ https://raw.githubusercontent.com/stacktracejs/stacktrace-gps/master/dist/stackt
 
 #### `new StackTraceGPS(/*optional*/ options)` => StackTraceGPS
 options: Object
-* **sourceCache: Object (String URL => String Source)** - Pre-populate source cache to avoid network requests
+* **sourceCache: Object (String URL : String Source)** - Pre-populate source cache to avoid network requests
+* **sourceMapConsumerCache: Object (Source Mapping URL : SourceMap.SourceMapConsumer)** - Pre-populate source cache to avoid network requests
 * **offline: Boolean (default false)** - Set to `true` to prevent all network requests
 * **ajax: Function (String URL => Promise(responseText))** - Function to be used for making X-Domain requests
 * **atob: Function (String => String)** - Function to convert base64-encoded strings to their original representation