Change license to MIT (r-lib#1277)

Closes r-lib#1252

Author: batpigandme

DESCRIPTION

@@ -18,7 +18,7 @@ Description: Automate package and project setup tasks that are
     otherwise performed manually. This includes setting up unit testing,
     test coverage, continuous integration, Git, 'GitHub', licenses,
     'Rcpp', 'RStudio' projects, and more.
-License: GPL-3
+License: MIT + file LICENSE
 URL: https://usethis.r-lib.org, https://github.com/r-lib/usethis
 BugReports: https://github.com/r-lib/usethis/issues
 Depends: 

LICENSE

@@ -0,0 +1,2 @@
+YEAR: 2020
+COPYRIGHT HOLDER: usethis authors