docs(README, MIGRATION): Update README, add MIGRATION-GUIDE (#28)

- Update README file to reflect the changes of the new major version
- Add a MIGRATION-GUIDE, explaining the breaking changes and their way of migration more detailed

Author: itsdevdom

MIGRATION-GUIDE.md

@@ -0,0 +1,63 @@
+# Migration Guide
+
+Strictly following the principle of semantic versioning, breaking changes only occur between major versions. This Migration Guide gives
+developers a more detailed insight into the changes introduced with new major releases, in particular the breaking changes and their
+consequences, while also suggesting a migration strategy.
+
+Also see then **[CHANGELOG](./CHANGELOG.md)** and **[GitHub releases](https://github.com/dominique-mueller/angular-notifier/releases)**.
+
+<br>
+
+## Migration from `1.x` to `2.x`
+
+> The amount of breaking changes from `1.x` to `2.x` is rather small, a migration shouldn't take longer than 5 minutes.
+
+#### Compatible with Angular 4+ only
+
+The library is now compatible with Angular 4+, using the all new & improved Angular APIs (such as the new `Renderer2`). Consequently, this
+also means that the compatibility with Angualr 2 breaks. If you still want to stick to Angular 2, you can continue using the latest `1.x`
+release; however, all new development (inlcuding bug fixes and features) will happen in the new `2.x` releases.
+
+#### Renaming of component selectors and classes
+
+For consistency reasons, all component selectors and CSS class names got renamed to no longer use the `x-` prefix. To migrate your
+application, simply rename the `x-notifier-container` tag to `notifier-container`. Also, if you did write custom themes or overwrote the
+default styling, you should remove the `x-` prefix from all CSS class names. The SASS variables, however, are still named the same.
+
+#### Renaming of module `forRoot()` method
+
+The `NotifierModule.forRoot()` method was used for passing custom options to the notifier. While the functionality stays the exact same, the
+method is now called `NotifierModule.withConfig()` instead. This seemed to be the more semantic, meaningful name here.
+
+#### Names & paths of published files
+
+With Angular 4+, a new recommendation regarding the publishment of Angular libraries has been defined. This includes a different folder
+structure, and also different output files. Therefore, the published files now include:
+
+- `angular-notifier.js` as the "rolled up" ES6 FESM (Flat ECMAScript Module) bundle
+- `angular-notifier.es5.js` as the "rolled up" ES5 FESM (Flat ECMAScript Module) bundle, however using ES6 import
+- `angular-notifier.umd.js` as the ES5 UMD (Universal Module Definition) bundle, here for compatibility reasons
+- Both the original `styles.scss` and compiled `styles.css` file exist, yet are available at the root path; sub-files are now located in the
+  "styles" folder
+- Also, the places of all the sourcemaps and TypeScript definition files changed (which, however, shouldn't affect anyone)
+
+*The only change affecting developers is probably the path change of the SASS / CSS files. When using SystemJS, a path change of JavaScript
+files might also be necessary. Most modern frontend build tools (such as Webpack or Rollup) will recognize and understand this library and
+its published files automatically.*
+
+#### Web Animations API polyfill
+
+The implementation of animations has been changed slightly, so that now the *default* Web Animations API polyfill should be sufficient to
+make this library work in older browsers. This is also the polyfill defined within Angular CLI based projects in the `polyfills.ts` file by
+default. While it for sure will save us a few bytes over the network line, it also prevents confusion amongst developers (such as
+**[#6](https://github.com/dominique-mueller/angular-notifier/issues/6)**,
+**[#10](https://github.com/dominique-mueller/angular-notifier/issues/10)**). In particular:
+
+``` typescript
+// With 1.x
+import 'web-animations-js/web-animations-next.min.js';
+
+// Now with 2.x
+import 'web-animations-js';
+// Same as: import 'web-animations-js/web-animations.min.js';
+```