[Website] Blog post announcing R package release

[asfimport]

After the R package makes it to CRAN, we should announce it.

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

Related issues:

• [R] Enable package to be made available on CRAN (depends upon)

PRs and other links:

• GitHub Pull Request #4948

_{Note: This issue was originally created as ARROW-6041. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4948
#4948