Add an overall module docstring

[Junxiao-Hou]

I added an overall module docstring to CSV. In this module docstring, I introduced the main use of CSV module, and the major CSV functions intended for reading and writing delimited text files. At the end, two brief examples of using CSV.read and CSV.write are provided. Now, when users type help?> CSV in Julia REPL, they can have a quick view/reminder of what this module is capable of:

help?> CSV

search: CSV VSCodeServer

  CSV provides fast, flexible reader & writer for delimited text files in various formats.

  Reading:

    •  CSV.File reads delimited data and returns a CSV.File object, which allows dot-access to columns and iterating rows.

    •  CSV.read is similar to CSV.File but used when the input will be passed directly to a sink function such as a DataFrame.

    •  CSV.Rows reads delimited data and returns a CSV.Rows object, which allows "streaming" the data by iterating and thereby has a lower memory footprint than CSV.File.

    •  CSV.Chunks allows processing extremely large files in "batches" or "chunks".

  Writing:

    •  CSV.write writes a Tables.jl interface input (https://github.com/JuliaData/Tables.jl) such as a DataFrame to a csv file or an in-memory IOBuffer.

    •  CSV.RowWriter creates an iterator that produces csv-formatted strings for each row in the input table.

  Here is an example of reading a csv file and passing the input to a DataFrame:

  using CSV, DataFrames
  ExampleInputDF = CSV.read("ExampleInputFile.csv", DataFrame)

  Here is an example of writing out a DataFrame to a csv file:

  using CSV, DataFrames
  ExampleOutputDF = DataFrame(rand(10,10), :auto)
  CSV.write("ExampleOutputFile.csv", ExampleOutputDF)

[quinnj]

Thanks!