Docs: Update usage & add API documentation (closes #24) (#29)

README.md

@@ -14,24 +14,54 @@ Add/write sourcemaps to/from Vinyl files.
 
 ```js
 sourcemap.add(file, function(err, updatedFile) {
-
+  // updatedFile will have a .sourceMap property
 });
 
+// The 2nd argument can be given as a path string
 sourcemap.write(file, './maps', function(err, updatedFile, sourcemapFile) {
+  // sourcemapFile will be a Vinyl file to be written to some location
+  // updatedFile will have the .contents property updated with a sourceMappingURL that resolves to sourcemapFile
+});
 
+// If not defined, the sourcemap is inlined
+sourcemap.write(file, function(err, updatedFile, sourcemapFile) {
+  // sourcemapFile is undefined
+  // updatedFile will have the .contents property updated with a sourceMappingURL that is an inlined sourcemap
 });
 ```
 
 ## API
 
 ### `sourcemap.add(file, callback)`
 
+Takes a [Vinyl][vinyl] `file` object and a `callback` function. It attempts to parse an inline sourcemap or load an external sourcemap for the file. If a valid sourcemap is found, the `sources` & `sourcesContent` properties are resolved to actual files (if possible) and a fully resolved sourcemap is attached as `file.sourceMap`. If a sourcemap is not found, a stub sourcemap is generated for the file and attached as `file.sourceMap`.
+
+Once all resolution is complete, the `callback(err, updatedFile)` is called with the `updatedFile`. If an error occurs, it will be passed as `err` and `updatedFile` will be undefined. __Note:__ The original file is mutated but `updatedFile` is passed to the callback as a convenience.
+
+If the `file` is not a Vinyl object or the contents are streaming, an Error will be passed to the `callback`.
+
+If the `file` has a `.sourceMap` property or the contents are null, the `callback` will be called immediately without mutation to the file.
+
+All filesystem operations are optional & non-fatal so any errors will not be bubbled to the `callback`.
+
 ### `sourcemap.write(file, [outputPath,] callback)`
 
+Takes a [Vinyl][vinyl] `file` object, (optionally) an `outputPath` string and a `callback` function.
+
+If `outputPath` is not passed, an inline sourcemap will be generated from the `file.sourceMap` property and appended to the `file.contents`. Once the inline sourcemap is appended, the `callback(err, updatedFile)` is called with the `updatedFile`. If an error occurs, it will be passed as `err` and `updatedFile` will be undefined. __Note:__ The original file is mutated but `updatedFile` is passed to the callback as a convenience.
+
+If `outputPath` is passed, a new Vinyl file will be generated using `file.cwd` and `file.base` from the original file, the path to the external sourcemap, and the `file.sourceMap` (as contents). The external location will be appended to the `file.contents` of the original file. Once the new file is created and location appended, the `callback(err, updatedFile, sourcemapFile)` is called with the `updatedFile` and the `sourcemapFile`. If an error occurs, it will be passed as `err` and `updatedFile`/`sourcemapFile` will be undefined. __Note:__ The original file is mutated but `updatedFile` is passed to the callback as a convenience.
+
+If the `file` is not a Vinyl object or the contents are streaming, an Error will be passed to the `callback`.
+
+If the `file` doesn't have a `.sourceMap` property or the contents are null, the `callback` will be called immediately without mutation to the file.
+
 ## License
 
 MIT
 
+[vinyl]: https://github.com/gulpjs/vinyl
+
 [downloads-image]: http://img.shields.io/npm/dm/vinyl-sourcemap.svg
 [npm-url]: https://npmjs.com/package/vinyl-sourcemap
 [npm-image]: http://img.shields.io/npm/v/vinyl-sourcemap.svg