Update package documentation about dbparser HOT 3 CLOSED

A note from ropen sci

I think the "Exploring the data" section is a little too heavy for README. This kind of contents is great in the getting started vignette but is not that useful in README as it is basically demonstrating dplyr and ggplot.

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• I think it would help unfamiliar users to add another sentence in the introduction explaining what DrugBank is. It would also be helpful to include a note about the need to make an account and request access to the data.
• Suggest removing commented code
• It seems like you made some changes recently that are resulting in errors in the README, so I suggest combing through it to make sure it's working as expected. Below are a couple things I've noticed.
• parse_drug_targets_actions() no longer works (it's now parse_drug_targ_actions()). Side note: I agree with @haozhu233 it would be nice to shorten some of the function names if possible.
  xml_db_name is not an argument in open_db()
• As expected parse_drug(save_table = TRUE) (and all parse functions) throws an error when there is no database to save to. However, the error only happens after parsing the data for about a minute, so it would be nice to throw an early informative error to the user.
• Make sure all relevant libraries are called for the plots (ggplot2, dplyr)
• The package description in R help can use a bit of clarification. I'd suggest emphasizing the drug bank parser over the database functionality.

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Enhance db connection section as note below

Database
This may be due to inexperience on my part, but I was confused about how to save the database using the driver and server argument. I would have liked to be able to save it to a SQLite or other local database using DBI::dbConnect. Is it possible to make these functions more generalizable? Or at minimum, add more description of what type of databases the package supports?

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