Improve the Linux setup documentation

[steverabino]

As a newbie to Linux myself, I found it pretty difficult to set up, and ended up doing more things than I needed to do.

This change attempts to split out the Linux setup steps into different pieces of functionality so users can decide if they need that function, or live without it. It also adds instructions of how to set up a daemon, which comes free with Mac.

I've tried to not be too prescriptive with these steps as I'm aware that I have limited exposure to different Linux distros, so would be happy to change if there's anything here that's deemed "too ubuntu"!

Mostly following advice on #155

[nateberkopec]

Would love a review from anyone using puma-dev on Linux.

[YoranBrondsema]

For Chrome:

1. Settings
2. Advanced --> Privacy and security
3. Manage certificates
4. Open tab Authorities
5. Import
6. Select file ~/.puma-dev-ssl/cert.pem

[steverabino]

I decided against putting specific instructions for this step:

• there are multiple ways to do this (adding to the operating system trust store vs. doing so in a specific browser), and the complexities around which browsers use/do not use the operating system store
• multiple browsers (should I add instructions for Firefox, Opera, etc)
• changes that will occur between browser versions.

I will add a bit more details about the options of using the operating system certificate trust store vs. adding directly to the browser which should give enough information to the user to be able to search online for information

[YoranBrondsema]

I just followed the steps and managed to set up puma-dev on Ubuntu 18.04. It works great and thanks to the system daemon, it also works between computer restarts.

I added a small comment on how to add the CA to Chrome.

Thanks a bunch @steverabino! Very timely PR.

[nateberkopec]

Let's address @YoranBrondsema's comment and then I'll merge. Thanks for the review Yoran!