Add a contributing guide

[Fcmam5]

As newbie, I'd like to help but I'm little bit lost I don't know where I can add/edit properties.

[roryokane]

Here’s some text the Contributing guide could include:

The [`property/`](https://github.com/jgthms/css-reference/tree/master/property) directory contains the HTML descriptions of each property. For example, the explanation and examples for the [`align-content`](http://cssreference.io/property/align-content/) property is in [`property/align-content/index.html`](https://github.com/jgthms/css-reference/blob/master/property/align-content/index.html).

The property/ directory contains the HTML descriptions of each property. For example, the explanation and examples for the align-content property is in property/align-content/index.html.

However, I am confused by seeing that the text in those HTML was apparently generated from Markdown, yet I don’t see any Markdown source files in the repo. It would be nice if the contributing guide either pointed to the Markdown files or explicitly said “while Markdown was used for the initial build, changes from now on should be made in the raw HTML”.

[yuezk]

@roryokane I think it was written in HTML, not markdown. But it missed some build scripts. For example, how to generate files in _includes/collections or _includes/lists?

[roryokane]

@yuezk Here is why I think the text was originally written with Markdown:

• _config.yml contains markdown: kramdown
• a typo involving ** double-asterisks caused some text to be bolded incorrectly

[yuezk]

@roryokane，you are right. 👍