docs: add Mailgun instructions

[TomGranot]

Just updating the Mailgun instructions for Heroku as per this thread.

[asbiin]

I think the mailgun secret can be grab from this address: https://app.mailgun.com/app/account/security (Private API Key field).

[asbiin]

@Tomgs Great job otherwise!

[TomGranot]

Hey @asbiin I think that's what I wrote - is it not clear from the instructions?

[asbiin]

@Tomgs you wrote

* Get your API key by following [these instructions](https://help.mailgun.com/hc/en-us/articles/203380100-Where-can-I-find-my-API-key-and-SMTP-credentials-).

I say we can help the user find the key by adding this link that display the link directly: https://app.mailgun.com/app/account/security

[TomGranot]

@asbiin You mind adding it? I'm not in front of my dev machine now

[asbiin]

Thanks @Tomgs

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