Add information about updating the client to README.md

[ysemennikov]

Hi!

Following the issue #1057, I suggest to add some information to README.md, so it will be easier to update the client.

Here is what I found in CHANGELOG.md:

The update command is no more, you can (mostly) replace its usage with some new flags on the generate command.

If you had a package named my-api-client in the current working directory, the update command previously would update the my_api_client module within it. You can now almost perfectly replicate this behavior using openapi-python-client generate --meta=none --output-path=my-api-client/my_api_client --overwrite.

The only difference is that my-api-client would have run post_hooks in the my-api-client directory,
but generate will run post_hooks in the output-path directory.

Alternatively, you can now also run openapi-python-client generate --meta=<your-meta-type> --overwrite to regenerate
the entire client, if you don't care about keeping any changes you've made to the generated client.