Prepare for a new version

[asinghvi17]

This is a combined PR for a bunch of different PRs that are currently up. Below is a summary of changes:

• [X] Add metadata to the dataframe returned by dataset, indicating that the dataframe was generated by RDatasets.jl and mentioning its package and dataset name as a Tuple. This is essentially a call DataFrames.metadata!(df, "RDatasets.jl" => (package_name, dataset_name)).
• [X] Add a description function to RDatasets, make it readable in the REPL
  • [X] Make this function discoverable, document it.
• [X] Bump RData.jl compat to 1.
• [X] Add instructions for data addition and improve data addition script
• [X] Bump version to v0.8

PRs #135 from @frankier and #124 from @jbrea are incorporated here.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 18.18182% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 63.88%. Comparing base (b1a5959) to head (4aac673).

Files	Patch %	Lines
src/dataset.jl	18.18%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #145 +/- ## =========================================== - Coverage 83.33% 63.88% -19.45% =========================================== Files 3 4 +1 Lines 24 36 +12 =========================================== + Hits 20 23 +3 - Misses 4 13 +9 ```

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[bkamins]

The changes look to make sense. I left one comment. I am not a maintainer of this package (and I do not know its internals). Maybe @nalimilan knows who has appropriate knowledge of the internals to approve it. Thank you for working on it.