Seal_of_Approval.md created and added to R build ignore

- Add link to Seal of Approval doc
- Add links to social media accounts and The Raft

Three packages proposed for Seal:

- nc:  By Toby, adds regular expressions to data.table
- tidyfast: By Tyson, implements dplyr-like verbs with only data.table as a dependency
- dtplyr: By Hadley, adds data.table backend to dplyr verbs.

Merging of this PR by a maintainer will consistute Approval on these three packages!