Document latest changes to the Homebrew OMERO formula (rebased onto dev_5_0)



This is the same as gh-635 but rebased onto dev_5_0.

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This PR documents changes brought to the Homebrew formula in ome/homebrew-alt#49

- Ice (3.5) is now the default version installed by `brew install omero`
- Add options to use old 3.3 and 3.4 versions
- Update PYTHONPATH setting in the configuration section

/cc @bramalingam