Update and Improve README.AIX

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-02-29.13:48:37.366>
labels = ['3.8', 'build', '3.7', '3.9']
title = 'Update and Improve README.AIX'
updated_at = <Date 2021-05-26.18:33:44.720>
user = 'https://github.com/isidentical'

bugs.python.org fields:

activity = <Date 2021-05-26.18:33:44.720>
actor = 'Ayushparikh-code'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Build']
creation = <Date 2020-02-29.13:48:37.366>
creator = 'BTaskaya'
dependencies = []
files = []
hgrepos = []
issue_num = 39798
keywords = ['patch']
message_count = 3.0
messages = ['362982', '363669', '364958']
nosy_count = 4.0
nosy_names = ['David.Edelsohn', 'Michael.Felt', 'BTaskaya', 'Ayushparikh-code']
pr_nums = ['26385']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue39798'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']