README.md: Make available on CRAN

Issue

The CRAN package page https://cran.r-project.org/package=backports has no instructions on how to use backports. A package developer will look for either a vignette or a README, but there's nothing.

Suggestion

Remove

backports/.Rbuildignore

Line 1 in 05e1381

^README.md$

so that the README.md file is bundled with the tarball, so that it ends up on CRAN.

Thanks!