doc: license year and R-Universe link

[hongyuanjia]

Pull request overview

• Update license year
• Include the default CRAN mirror in repos when installing the dev version

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.75%. Comparing base (6763e75) to head (2db389c).

:exclamation: Current head 2db389c differs from pull request most recent head 2b7d044. Consider uploading reports for the commit 2b7d044 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #84 +/- ## ======================================= Coverage 99.75% 99.75% ======================================= Files 8 8 Lines 2017 2017 ======================================= Hits 2012 2012 Misses 5 5 ```

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