Add a pkgdown site

[jameslamb]

We should add a pkgdown site hosted on Github pages! e.g. https://uptakeopensource.github.io/uptasticsearch/

Can be created by just running pkgdown::build_site().

Note that the pkgdown maintainers have made a lot of changes the last few weeks, so should update to the newest version before running this.

[jayqi]

#26 added the pkgdown site to the repo.

@cwschultz88 I think you need to go into the settings on GitHub for this repo to enable it. Also don't forget to add the link to the description blurb.