docs: Add clang-format instructions for mac

[jakesylvestre]

As per this stack overflow answer mac doesn't ship with clang-format.

I've added instructions for installing

Pull Request Checklist

• If this is your first time contributing, we recommend you read the Code
  Contribution Guidelines
• All changes are Go version 1.12 compliant
• The code being submitted is commented according to Code Documentation and Commenting
• For new code: Code is accompanied by tests which exercise both
  the positive and negative (error paths) conditions (if applicable)
• For bug fixes: Code is accompanied by new tests which trigger
  the bug being fixed to prevent regressions
• Any new logging statements use an appropriate subsystem and
  logging level
• Code has been formatted with go fmt
• Protobuf files (lnrpc/**/*.proto) have been formatted with
  make rpc-format and compiled with make rpc
• For code and documentation: lines are wrapped at 80 characters
  (the tab character should be counted as 8 characters, not 4, as some IDEs do
  per default)
• Running make check does not fail any tests
• Running go vet does not report any issues
• Running make lint does not report any new issues that did not
  already exist
• All commits build properly and pass tests. Only in exceptional
  cases it can be justifiable to violate this condition. In that case, the
  reason should be stated in the commit message.
• Commits have a logical structure according to Ideal Git Commit Structure

[guggero]

LGTM, thanks!

[carlaKC]

Nice 🥇