Adds topic/pharo

[eMaringolo]

Thank you for contributing! Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/master/CONTRIBUTING.md
• I have no affiliation with the project I am suggesting (as a maintainer, creator, contractor, or employee).

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288)
• Content (and my changes are in index.md)

Please replace this line with an explanation of why you think these changes should be made.

Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• My folder contains a *.png image (if applicable) and index.md
• All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/master/docs

Pharo is moving most of its repositories from a self hosted solution at smalltalkhub.com to github.com, and we think that having a topic for Pharo will help in the discovery of Pharo related projects.

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Please note: we will close your PR without comment if you do not check the boxes above and provide ALL requested information.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @eMaringolo!