Add header resources

In the interest of making this repository capable of reproducing the full
/r/writing stylesheet anywhere, add the header images into a separate
header/ directory. Adjust $header-height in style.scss to use the height
of the default logo.
Add flair icons

Similar to commit 67af58, add flair icons to the repository and make
filenames more predictable.
Add README

Add markdown README to serve as a basic guide for users unfamiliar with
SASS and to give credit to those who helped create this theme.