Add a syntax guide to use after migration to codemods

Migration.md

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+# After ES6 classes migration
+
+The purpose of this document is to provide a quick syntax reference to usage of ES6 classes to avoid some common mistakes.
+
+**Note** Any new code can be restricted to use only ES6 classes with the help of [eslint plugin](https://github.com/scalvert/eslint-plugin-ember-es6-class) after the [codemods](https://github.com/scalvert/ember-es6-class-codemod) are run.
+
+# Classes
+
+### Before
+```
+let MyComponent = Component.extend({
+
+});
+```
+### After
+```
+class MyComponent extends Component {
+
+}
+```
+## Default export
+### Before
+```
+export default Component.extend({
+
+});
+```
+### After
+```
+export default class MyComponent extends Component {
+
+}
+```
+**Note:** It is recommended to have class name for default export, though its not mandatory.
+
+## Mixins
+### Before
+```
+let MyComponent = Component.extend(AMixin, BMixin, {
+
+});
+```
+### After
+```
+class MyComponent extends Component.extend(AMixin, BMixin) {
+
+}
+```
+
+# Properties
+
+### Before
+```
+let someVal = "value";
+
+let MyComponent = Component.extend({
+  prop: "defaultValue",
+  boolProp: true,
+  numProp: 123,
+  [MY_VAL]: "val",
+  queryParams: {},
+  someVal
+});
+```
+### After
+```
+let someVal = "value";
+
+class MyComponent extends Component {
+  prop = "defaultValue";
+  boolProp = true;
+  numProp = 123;
+  [MY_VAL] = "val";
+  queryParams = {};
+  someVal = someVal;
+}
+```
+**Note** The property shorthand does not work in classes as in objects. A class property should be assigned a value otherwise it's value `undefined`. See class property `someVal` in above example.
+
+# Methods
+
+### Before
+```
+let someFunction = () => {};
+
+let MyComponent = Component.extend({
+  someUtilFunction() {
+    this._super(...arguments);
+    return value;
+  },
+
+  someFunction,
+
+  _someOtherFunction() {
+    return value;
+  }
+});
+```
+### After
+```
+let someFunction = () => {};
+
+class MyComponent extends Component {
+  someUtilFunction() {
+    super.someUtilFunction(...arguments);
+    return value;
+  }
+
+  someFunction = someFunction;
+
+  _someOtherFunction() {
+    return value;
+  }
+}
+```
+**Note** The `someUtilFunction` must be present in super class hierarchy. An error will be thrown otherwise at run time
+
+# Actions
+
+### Before
+```
+let someAction = () => {};
+
+let MyComponent = Component.extend({
+  actions: {
+    onButtonClick() {
+      this._super(...arguments);
+      return value;
+    },
+
+    someAction,
+  }
+});
+```
+### After
+```
+let someAction = () => {};
+
+class MyComponent extends Component {
+  @action
+  onButtonClick() {
+    super.actions.onButtonClick.call(this, ...arguments);
+    return value;
+  },
+
+  @action
+  someAction() {
+    someAction.call(this, ...arguments);
+  }
+}
+```
+**Notes**
+1. The `super` calls in actions are tricky. The `actions` are part of the actions hash on ember objects. So they can be referenced through `super.actions` object. We need to use `call` method to make sure they are bound to correct `this`.
+
+2. Property shorthand does not work correctly in actions. So we have to wrap it inside function. See the action `someAction` in the above example.
+
+3. Make sure that the `onButtonClick` is present in `actions` hash in super class hierarchy. An error will be thrown otherwise at run time
+
+# Decorators
+Usage of decorators is well documented in [ember decorators](http://ember-decorators.github.io/ember-decorators/)