add upgrading guide

UPGRADING.md

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+# Upgrading guide
+
+From 1.x to 2.x
+
+If you're coming from 1.x, there are a couple things that changed and will be presented here in the format OLD vs NEW.
+
+## Initializing
+
+```javascript
+// OLD
+var sharect = new Sharect()
+sharect.init()
+
+// NEW
+Sharect.init()
+```
+
+## Facebook SDK
+
+There is no need to add the Facebook SDK anymore, neither create an `appId`. Because of this, Facebook sharing option is visible by default. You can change this behaviour (simulating the old one) by setting it to `false`:
+
+```javascript
+Sharect.config({
+  facebook: false
+}).init()
+```
+
+## Selectable Elements
+
+You can now choose what elements can be selected and shared. [Read more in .config documention](/README.md#config).
+
+By default, it is `body`, the same behaviour as before.
+
+## Twitter username
+
+You don't need to include the `@` anymore, as we are now using the default Twitter API, which includes it.
+
+```javascript
+// OLD
+var sharect = new Sharect()
+sharect.config({
+  twitterUsername: '@estevanmaito'
+}).init()
+
+// NEW
+Sharect.config({
+  twitterUsername: 'estevanmaito'
+}).init()
+```