Added paragraph to readme noting config impacts on Magma build

[djlawren]

Description

Makes changes to the README to note incoming changes from this PR: https://github.com/mitre/caldera/pull/2994

Type of change

Please delete options that are not relevant.

• [X] This change requires a documentation update

How Has This Been Tested?

No tests needed.

Checklist:

• [X] I have made corresponding changes to the documentation