Update copyright notice

[ramiromagno]

• Add Copyright field to DESCRIPTION
• Remove Appendix from the Apache 2 License
• Add a NOTICE file as hinted by the text of the Apache License

Note that I'm aware that many R packages using the Apache 2 License do not make any of these changes that I am proposing here. I think they are important nonetheless, and invite you to read:

• Original Apache 2 License text: https://www.apache.org/licenses/LICENSE-2.0
• https://www.linkedin.com/pulse/how-apply-apache-20-license-your-open-source-software-vladim%C3%ADr-gorej/

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[ramiromagno]

@rammprasad , @galachad, @edgar-manukyan, @kamilsi, @ShiyuC, @yli110-stat697: I'm sure this PR is going to raise a nice heated debate, and I think it would be best to do it in a meeting, but also think that have something concrete to discuss about is also helpful, so here it is this commit / PR, and a few extra notes for your consideration:

Note that I'm aware that many R packages using the Apache 2 License do not make any of these changes that I am proposing here. I think they are important nonetheless, and invite you to read:

• Original Apache 2 License text: https://www.apache.org/licenses/LICENSE-2.0
• https://www.linkedin.com/pulse/how-apply-apache-20-license-your-open-source-software-vladim%C3%ADr-gorej/
• https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Licensing

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Package	Line Rate	Health
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Summary	93% (953 / 1021)	✔