Add instruction to generate an access token.

[Autive]

I was struggling to integrate this package cause in my mind the access token was pa_{Identification Code}.
Just later i discovered that creating a client make the needed access token, so adding an instruction maybe helps others.

[Autive]

@Kugelschieber maybe it would also be a big plus to mention "access tokens" in the Pirsch dashboard.
For me creating Clients to get an access token isn't really that obvious. So having an additional text similar to:

"Create clients to create an access token or oAuth credentials to access the API.
Your dashboard ID is XygJrQzg3w "

[Kugelschieber]

Thank you for the suggestion :) We can add it to the hint on the left.

[zepfietje]

I've made some final tweaks, but thanks for the PR, @Autive!