Add important notice about `chsh -s /bin/zsh` command requiring the user to log out and log back in to see the changes.

Introducing a Code of Conduct based on the Contributor Covenant project. Adding links from the README and CONTRIBUTING pages to this, too, along with an email address for reporting abusive behavior to. Closes #7733 (#7734)

* Updating Oh My Zsh shop URLs

Linking directly to the Oh My Zsh inventory vs the top-level store with non-OMZ items.

* Updating link to Oh My Zsh products in the install script

* Updating link to Oh My Zsh shop products in the upgrade script

* Getting rid of 't-' in shirts for now

The git:// transport is completely unauthenticated. An attacker on the local or upstream network can easily man-in-the-middle an oh-my-zsh update and get remote code execution on your system. Only the https:// git transport should be used.

Updating template and README to list plugins on individual lines vs one long one. Easier scanning for you and me.

* README.md: shit load -> shit-load

* README.md: shit-load -> shitload

Closes #5681

Closes #5558

https://github.com/robbyrussell/oh-my-zsh/wiki/Themes#agnoster

Change OS X to macOS in the read me file in line with apples rebranding
of the operating system.

From `raw.github.com' to `raw.githubusercontent.com'

Fixes #4933

Also, fix a couple of typos / style issues.

 - Adding documentation
 - Note the effect empty array
 - Fix spelling
 - Using new test `[[...]]`

The tilde character is not expanded when inside quotes. See https://github.com/robbyrussell/oh-my-zsh/issues/4668#issuecomment-162213379

Change http://ohmyz.sh link to use proper markdown syntax

Fix typo

A few spelling fixes, grammatical fixes (what's with all the ellipses?), some standardization of capitalization, and a few other tweaks.

As well, I changed all standalone code fragments into proper code blocks.

I also added a missing theme image!

Updating the install instructions as proposed in: https://github.com/robbyrussell/oh-my-zsh/issues/3516#issuecomment-76506726