Update README.md with note about where gdk is installed

[aws-kevinrickard]

Issue #, if available:

Description of changes:

I got tripped up when installing into a venv because the python bin was not on my path. This note helps avoid this pitfall and can be easily ignored for people who already understand how python installation works

Why is this change necessary:

How was this change tested:

Any additional information or context required to review the change:

Checklist:

• Updated the README if applicable
• Updated or added new unit tests
• Updated or added new integration tests
• Updated or added new end-to-end tests
• If your code makes a remote network call, it was tested with a proxy
• You confirm that the change is backwards compatible

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