[ENHANCEMENT] Add "Getting Started" documentation and Julia guidance

## Instructions

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### Relation

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### Description

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### Alternatives

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### Additional context

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## FILL ME OUT

### Relation

The organization is starting to support multiple software languages and there is a lack of guidance on how actually get started using some of the repos.

### Description

In [em-overview/README](https://github.com/Airspace-Encounter-Models/em-overview/blob/master/README.md), add a "Getting Started" section and provide guidance on how to get Julia installed.

### Alternatives

There are plenty of online resources on how to install Julia, so please don't reinvent the wheel. Please refer and link to these resources when appropriate.

### Additional

Although the CONTRIBUTING guidelines discourage documentation only issues, @aweinert-MIT approved this issue as Julia software was just git forked into the organization.