Add instructions on how to get the oauth url without the use of a browser extension

[FeikoJoosten]

Hi there,

I noticed you stated that you need to use the browser extension to get sent back to home assistant for the oauth callback. While it does make it a lot easier, you don't need to install the extension either.

If you open the browser console after having logged into post nl, you'll see an error that it couldn't launch an application. It also states the oauth url required. Manually copying that url, replacing the postnl://login section with https://my.home-assistant.io/redirect/oauth allows you to manually navigate to the newly formatted url. This is then caught by home assistant and the integration successfully installs. Alternatively, you can also enable the networking recorder and catch the oauth url that way. But I don't have example screenshots at hand for that.

Here are some example screenshots. First login to post nl

After loggin in you get this blank page

Open the browser console and find the url

Copy the url and replace the postnl://login section as stated before. Then manually navigate to the new url.