Document purpose of letter-spacing in .pure-g-r

[rob-mccann]

https://github.com/yui/pure/blob/master/src/grids/css/grids-r.css#L4-L6

This causes text to be bunched up on FF, setting letter spacing to normal fixes it across browsers.

What's the reason for this being here?

[lkraav]

Read #41

This is the whole backbone of the inline-block grid system.

[rob-mccann]

It'd be good to have a link to #41 in the source code - a couple of us were confused as to it's purpose when we looked at source code.

I'll create a pull request if needed?

[ericf]

@rob-mccann this is documented in various places:

• https://github.com/yui/pure/blob/v0.3.0/HISTORY.md#grids
• https://github.com/yui/pure/blob/v0.3.0/src/grids/css/grids-core.css#L3-40
• http://purecss.io/grids/#introduction-to-pure-grids
• http://blog.purecss.io/post/60789414532/how-we-improved-grids-in-pure-0-3-0

Feel free to open a PR if you want to improve them.