docs(DEVELOPER.md): state that git-clang-format must be in PATH

[Mischi]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  docs update
• What is the current behavior? (You can also link to an open issue here)
  To use git clang-format your have to make sure that git-clang-format is in your path.
  This PR adds a small NOTE so that people are aware of that fact.

[IgorMinar]

thanks

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