[Rd] Add keywords to descriptions of R packages

[beii@vsky m@iii@g oii @oi@com]

It's great that there are almost 20,000 R packages and that experts have curated the R packages in task views, but the volume of packages is such that task view maintainers may limit additions because of time constraints or because they don't want the task views to become unwieldy. I suggest that keywords, chosen by package authors from an approved list, be allowed in package descriptions. The keywords could be the same as task view names (for example "timeseries") or they could be more specialized (for example "garch"). Users would be able to search packages by keyword. Ideally, all R packages would be indexed and discoverable this way.
I maintain a list of time series packages that were not in the task view at the time I added them: https://github.com/Beliavsky/R-Time-Series-Task-View-Supplement/blob/main/README.md. There are almost 130 entries. I also have one for finance: https://github.com/Beliavsky/R-Finance-Task-View-Supplement .
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