Add docs for import / export feature

[Mozzo1000]

[..] I would like to know if the software supports Python plugins like Calibre [..]

Originally posted by @2380757563 in #57

BookLogr currently supports exporting to CSV, HTML and JSON file. You can import from the exported CSV file or from a Goodreads CSV file.

[Mozzo1000]

Also add technical docs for BookLogrs own csv file so it can be created by hand or by third-party tools.