STM32: update readme with clock information

[jeromecoutant]

Summary of changes

Quick addition in the STM32 readme file about clock settings

fix #14082

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@jeromecoutant, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[ciarmcom]

This pull request has automatically been marked as stale because it has had no recent activity. @ARMmbed/mbed-os-maintainers, please complete review of the changes to move the PR forward. Thank you for your contributions.

[0xc0170]

CI started