Install docs improved

[humitos]

Closes #2281
Related to #1060

What I did?

• added a note regarding elasticsearch installation for full search support
• added a note to mention about how to install postgresql and redis.
• removed two extra whitespaces
• move homebrew link together with the others

Possible missing things:

• instructions for OS X users (I have no idea!)

Let me know what else is missing here of if you want to write this in another way :)

[agjohnson]

Noted a change, but looks good otherwise.

[agjohnson]

You don't need Postgres actually, we don't have any Postgres-specific code that I know of. The development database is fine, but not recommended for production instances.

We do require Redis currently, though only because of a feature that prints our build queue in our templates. I think we could get rid of this however -- I'll open a ticket to discuss this. I think it's fine to leave the docs on redis in for now.

[ericholscher]

These are good changes. One thing I'd like to actually do is move a lot of the warnings and other cruft into a FAQ/other support type page, or at least put them at the bottom of the page. The install instructions currently are a bit esoteric in a lot of ways, and we should likely just be documenting the common path on Linux/OSX.

That's another PR though, this one looks good after Anthony's feedback.

[humitos]

I agree with @ericholscher. As I commented you, I think there is a needed another section called Tips & Tricks or something like that talking about the docker images, permission errors, etc.

I think it's ready to be merged now. I did the change that @agjohnson mentioned.