Add an example readme

docs/use-cases/data-residency-set-hostname.md

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+# Choosing a hostname to send messages to
+
+Use the `setDataResidency` setter to specify which host to send to:
+
+Send to EU (hostname: `https://api.eu.sendgrid.com/`)
+```js
+const sgMail = require('@sendgrid/mail');
+sgMail.setDataResidency('eu');
+const msg = {
+  to: '[email protected]',
+  from: '[email protected]',
+  subject: 'Hello world',
+  text: 'Hello plain world!',
+  html: '<p>Hello HTML world!</p>',
+};
+sgMail.send(msg);
+```
+Send to Global region, this is also the default host, if the setter is not used
+(hostname: `https://api.sendgrid.com/`)
+```js
+const sgMail = require('@sendgrid/mail');
+sgMail.setDataResidency('global');
+const msg = {
+  to: '[email protected]',
+  from: '[email protected]',
+  subject: 'Hello world',
+  text: 'Hello plain world!',
+  html: '<p>Hello HTML world!</p>',
+};
+sgMail.send(msg);
+```
+
+## Limitations
+
+1. Emails can only be sent to two hosts for now; 'eu' (https://api.eu.sendgrid.com/) and 'global' (https://api.eu.sendgrid.com/)
+2. The default hostname is https://api.sendgrid.com/
+3. The valid values for `region` in `client.setDataResidency(region)` are only `eu` and `global`. Case-sensitive.