Add docs that pyswarms is minimization solver

[yasirroni]

Describe the bug
No mention in both codes and docs that pyswarms optimize is minimization.

To Reproduce
run example of any maximization problems.

Expected behavior
Clear docs that maximization problems needs to be reformulated

Describe alternatives you've considered
Support maximization as parameter is optimize

[yasirroni]

Also, docs about the shape of x in this example:

# import modules
import numpy as np

# create a parameterized version of the classic Rosenbrock unconstrained optimzation function
def rosenbrock_with_args(x, a, b, c=0):
    f = (a - x[:, 0]) ** 2 + b * (x[:, 1] - x[:, 0] ** 2) ** 2 + c
    return f

should be clarified.