Add blogpost on new instance url helper (#498)

blog/2020-05-08-instance-url-helper.md

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+---
+author: Franck Nijhof
+authorURL: https://twitter.com/frenck
+authorImageURL: /img/profile/frenck.png
+authorTwitter: frenck
+title: Instance URL helper
+---
+
+If you are an integration developer and came across the problem of getting the
+URL of the users' Home Assistant instance, you probably know, this wasn't always
+easy.
+
+The main problem is that a Home Assistant instance is generally installed, at home.
+Meaning the internal and external address can be different and even those can
+have variations (for example, if a user has a Home Assistant Cloud subscription).
+
+Matters become worse if the integration has specific requirements for the URL;
+for example, it must be externally available and requires SSL.
+
+As of Home Assistant Core 0.110, a new instance URL helper is introduced to
+ease that. We started out with the following flow chart to solve this issue:
+
+[![Flow chart of getting a Home Assistant instance URL](/img/en/blog/2020-05-instance-url-helper/flowchart.png)](/img/en/blog/2020-05-instance-url-helper/flowchart.png)
+
+As a result of this, the previously available `base_url` is now replaced by two
+new core configuration settings for the user: the internal and external URL.
+
+From a development perspective, the use of `hass.config.api.base_url` is now
+deprecated in favor of the new `get_url` helper method.
+
+For more information on using and implementing this new URL helper method,
+consult our documentation [here](/docs/instance_url).