No mention of "sort_by" argument of get_grades() in README

[maciejkrol18]

In the README, the usage of get_grades is shown as follows:

from librus_apix.grades import get_grades

grades, average_grades = get_grades(token)

for mark in grades["Mathematics"]:
    print(mark.grade)

Running this code results in TypeError: get_grades() missing 1 required positional argument: 'sort_by'

[RustySnek]

The sort_by argument is for getting new grades or grades since last login. I actually forgot about it when writing the README. I will set the sort_by to get all grades by default and update the README file.