Improve README by adding:

• Information on usage (from (https://github.com/jennybc/here_here)[Ode to the here package]).
• Badges (CRAN and lifecycle)
• Allison Horst here illustration

Also added a "Getting Started" vignette which contains most of the same information as the README, but also further details on using here (and why we want to) in an Rmd in a subdirectory.

I assume you'll want to re-render the README so that it doesn't contain the file path to my tidy dev day folder. I'm a little fuzzy on where things (especially the pkgdown docs) might be off since@krlmlr has been making changes to the package while i've been working on it, but happy to work through it.

Closes#42