Readme updates based on setup feedback

[Andy George]

After an internal walkthrough by @cmgorton, we have some feedback to make some things a little clearer in the setup docs:

• add explicit pip3 install ansible step
• add ssh-add step after ssh-keygen
• separate the quick setup steps into macOS vs Linux
• add notes to each provider that let you know where your Forem VM lives at (looking at you, AWS EC2)
• add note about Digital Ocean "Authentication contexts" to make that clearer

[Christina]

After some feedback from Mac:
On step 7 of Quickstart guide add a note:
"If you use AWS, you will need to generate an RSA-based SSH key and save it to the file path ~/.ssh/id_rsa.pub"

This is mentioned in the AWS section but since people are working on step 7 before going to the Cloud providers some people are creating an ssh key and then seeing that they have to create a RSA-based one if they use AWS.

[Christina]

I created a pull request for the SSH section.
#34

[Christina]

Did you want me to try to update any of the other things here @andygeorge or did you get it already?

I could probably do:

• add note about Digital Ocean "Authentication contexts" to make that clearer
• add notes to each provider that let you know where your Forem VM lives at (looking at you, AWS EC2)

[Andy George]

@cmgorton if you're bored and want to knock those out, go for it! I am working on some other stuff at the moment, but will get to these ASAP.

[Leo Giovanetti]

I would also suggest explaining the automated steps to register a domain in the cloud provider such as DigitalOcean where the domain name gets registered alongside DNS setup. It would be great if that part could be optional, as I have setup my domain to work with Cloudflare and I couldn’t make my self hosted Forem work without removing DigitalOcean domain setup due to that previous configuration I already had.