Add note in readme suggesting `pip install -U --pre zowe.core-for-zowe-sdk[secrets]`

Is your feature or enhancement request related to a problem or limitation? Please describe

On the PyPI page, we mention the optional secrets dependency for the core package, but we do not mention that step in the readme at the top level of this repo, which can lead users to wonder why their secure creds are not being loaded into a profile dict when using ProfileManager().load() from the core sdk.

Describe your enhancement idea

It would be helpful to include the command to install the secrets package in the top level readme in this repository (pip install -U --pre zowe.core-for-zowe-sdk[secrets]).

Describe alternatives you've considered

The alternative would be to have the bundle install the secrets package by default. Is there some reason not to include the secrets package in the bundle?

Provide any additional context

Open the repository's top-level README and locate the installation guidance for the core SDK and its optional secrets dependency. Add the command pip install -U --pre zowe.core-for-zowe-sdk[secrets], making clear that it is needed for secure credentials used by ProfileManager().load(). Done means the command and purpose are visible in the README.