Documentation updates

Hi,

I think the documentation could use some updating. Willing to work on this if there is interest from maintainers. Is this the case? I would need some assistance in clearing things up then. Anyway , quick scan below.

Filed this under documentation component, but parts of this issue are UX too.

In the readme.md :
- is there a reason to mention mailchimps' competitors? Also this line crosses the 80 char border.
- I do not know for sure if all the sub-module texts are still accurate. But automation sub-module is not for D8 right?
- 'This module has not yet been ported to Drupal 8' , strange for a README.md in the d8-1.11 version.
- section about MCC should be removed, this is not a viable alternative (not even for D7 I believe, let alone D8. see #2118755: Is mcc (mailchimp campaigns) still being maintained?

Mailchimp settings page (/admin/config/services/mailchimp) :
- I don't think the description and helptext for the 'Connected Sites' settings are really clear and believe they could use some improvement. Could not get it working easily with the instruction provided.
At the least a link should point to https://mailchimp.com/help/about-connected-sites/#Connect_your_custom_we... anchor instead of the main page. If I am guessing correctly, not sure as I did not get it to work.
- Also the text for the webhook is a bit obscure

Submodules:
The readme.txt should at least be changed to readme.md , no?

Also #3049218: Change the documentation and UI to match naming changes w/ the API ? Or keep it a separate issue?
Same for #2988573: missing documentation for mailchimp_subscribe()/mailchimp_unsubscribe() ?

Love to hear what the maintainers thinks about this all.