Document how to import and use a custom tzif file.

[devries]

I provided the following response in a discussion, which we agreed should be part of the overall documentation when asked about using your own tzif file:

If you read the file with simplifile.read_bits you can get a bit array, then parse it with the tzif parser.parse command, and you can create a new timezone database using database.new and then add the loaded parsed file using database.add_tzfile giving the time zone a name. After that you can use it like a regular TzDatabase object, you can even start with the database loaded from the operating system and add new timezone definitions to it with the database.add_tzfile function.