Add pointer to .d.ts and mention addon CSS

Part of #1554

README.md

@@ -74,6 +74,8 @@ var xterm = new Terminal();  // Instantiate the terminal
 xterm.fit();                 // Use the `fit` method, provided by the `fit` addon
 ```
 
+You will also need to include the addon's CSS file if it has one in the folder.
+
 #### Importing Addons in TypeScript
 
 There are currently no typings for addons if they are accessed via extending Terminal prototype, so you will need to upcast if using TypeScript, eg. `(<any>xterm).fit()`.
@@ -107,6 +109,10 @@ Since xterm.js is typically implemented as a developer tool, only modern browser
 
 Xterm.js works seamlessly in Electron apps and may even work on earlier versions of the browsers but these are the browsers we strive to keep working.
 
+## API
+
+The current full API documentation is available in the [TypeScript declaration file on the repository](https://github.com/xtermjs/xterm.js/blob/master/typings/xterm.d.ts), switch the tag (press `w` when viewing the file) to point at the specific version tag you're using.
+
 ## Real-world uses
 Xterm.js is used in several world-class applications to provide great terminal experiences.