Add context to why ngrok is needed

[iteles]

As per https://github.com/dwyl/dwylbot/pull/166, we need to add context as to why ngrok is required/used.

[SimonLab]

This issue is linked to the section on how to run locally on your machine dwylbot: https://github.com/dwyl/dwylbot#run-the-project-locally. This section needs a bit more details with clear steps which should answer at least the following questions:

• Why would I want to run dwylbot locally?
• why I need to create my own Github App?
• Why I need to install ngrok?

The screenshots on this section need also to be updated (smaller and better quality):

[nelsonic]

@SimonLab just in case we want to link to a super-quick ngrok tutorial, "threw" this together: https://github.com/dwyl/learn-ngrok 😉