Update README, API docs prior to release

Since the 4.0 Python SDK is built with couchbase++ and txns++ as dependencies, whenever a user is cloning the repository they need to make sure to either clone the repo pulling in the dependencies, or use the correct submodule command to update the repo.  These commands/paths should be documented (use cb++ as template).
 

This repo uses several git submodules. If you are fetching the repo for the first time by command line, the

`--recurse-submodules` option will init the submodules recursively as well:

```shell

git clone --recurse-submodules https://github.com/couchbase/couchbase-python-client.git

```

However, if you fetched using a simple clone command (or another IDE or tool) **you must also perform** the following

command to recursively update and initialize the submodules:

```shell

git submodule update --init --recursive

```

Also, update README and API docs with the following:

• Added MacOS wheels
• Working on manylinux wheels
• Twisted v 21.7.0 as min version
• A note for older linux versions (specifically Centos/RHEL) on needing to update components if they do not meet the minimum requirements.