Add config documentation

[telegraphic]

Need to say: 1) what is optional, mandatory 2) What options mean

config = {
    'preprocess': {
        'sk_flag': True,
        'normalize': True,
    },
    'sk_flag': {
        'n_sigma': 3,
    },
    'dedoppler': {
        'boxcar_mode': 'sum',
        'kernel': 'ddsk',
        'max_dd': 4.0,
        'min_dd': None,
        'apply_smearing_corr': False,
        'beam_id': 0
    },
    'hitsearch': {
        'threshold': 3,
        'min_fdistance': 100
    },
    'pipeline': {
        'n_boxcar': 4,
        'merge_boxcar_trials': True,
        'n_blank': 2
    }
}