Update README.md

Expand installation description.
Merge pull request #53 from juanpastas/patch-1

Update README.md
Also allow importing sheets that take root in assets/
Remove selective asset paths entirely.
Tweak example text.
Tweak another example to check all paths.
Merge pull request #54 from Ideasaurus/import-from-non-stylesheet-dir

Allow @import to work with files located in assets/ (rather than only stylsheets/)
Test implementation of assets helpers, no tests.
resolve #52. Implementation of asset_path and asset_url helpers.