Add list of most useful abbreviations

[iteles]

When I first started using tachyons I found that there was quite a long list of things I kept having to look up. It would be good to have a 'starter crib sheet' for the most used abbreviations and their meanings across tachyons.

A good example of this is here: https://github.com/dwyl/learn-tachyons/blame/master/README.md#L326

[nelsonic]

@iteles this would be a very useful addition for beginners! 🎉

[iteles]

Some that aren't making it into the 'beginners' round but are also pretty useful (things like whitespace for example didn't make it onto this list and can be easily looked up):

• measure limits line length to what is generally agreed to be a readable line length
• tracked should only be used for all caps text and ensures readable letter spacing
• There are a number of font families already set up in tachyons with appropriate fallbacks but this module should be extended
• fs-normal forces a standard font style
• ttx transforms lettering to uppercase (ttu), lowercase (ttl) or capitalises the first letter (ttc)
• [.debug] adds a 1px gold border to all elements which are given this class
  • For a quick and more widespread debug, uncomment out the debug module line at the bottom of src/tachyons.css and then run node run build from the root of the project
• border-box is applied by default to all elements where this makes sense (making a declared width the actual width of the element, regardless of padding or borders added)
• Basic grids are super easy in tachyons and this page gives you some simple examples
• An absolutely positioned element can fill its parent relatively positioned element by using the class absolute--fillin conjunction with the absolute class (this also supports all of the standard tachyons break points)
• Several options are provided for box shadow, from shadow-1 to shadow-5 (note that the variation in these are not shadow sizes but shadow positions)
• pointer adds a pointer to the element on hover

[iteles]

Not sure whether the above should make it into our readme or not, maybe redundant (i.e. people have read the basics, now they want to read the full docs).