Basic documentation in README.md

Upon downloading new software, the first point of call for many people is to look for some help on how to install and use the software. This is where a file such as INSTRUCTIONS, INSTALL or README can come in handy. The name of the file says what it is and will attract the attention of the user. While these files can be in any format, and have any name, a common choice is README.md. If you have navigated to a GitHub software repository, and seen the nice documentation available on the front page, then this has been generated from the README.md file.

The markdown format (guide here) is a simple to use, future proof, platform independent, document format that can be rendered into a range of other formats. As a bonus, the raw files are easily read and written by humans.

Things to consider for your README.md file

• The name of the project
• A description of the purpose of the software
  • Maybe a one liner for each script
• Install instructions
  • List some high level dependencies
• Usage instructions
  • If you have a CLI then the output of mycode --help is appropriate to include verbatim
• A link to documentation
• Author information and contact details (email, or just a link to github issues)
• A note on how people should credit this work

README.md for our example project

# SkySim
This project was built in order to simulate source (star/galaxy/other) positions over an area of sky.

## Installing
This project relies only on python built-ins and the numpy library.
Use ```pip install -r requirements.txt``` if you don't yet meet these requirements.

## Usage
The main entry point for this project is `sim_catalog`:
```
./sim_catalog --help
usage: sim [-h] [--ref_ra REF_RA] [--ref_dec REF_DEC] [--radius RADIUS] [--n NSOURCES] [--out OUTFILE]

optional arguments:
  -h, --help         show this help message and exit

  --ref_ra REF_RA    Central/reference RA position HH:MM:SS.S format
  --ref_dec REF_DEC  Central/reference Dec position DD:MM:SS.S format
  --radius RADIUS    radius within which the new positions are generated (deg)
  --n NSOURCES       Number of positions to generate
  --out OUTFILE      Filename for saving output (csv format)
```

## Documentation
Documentation is currently just this file, and associated python docstrings.

## Author / Contribution
This project is developed by Dev One.
If you want to contribute to this project please create a fork and issue pull requests for new features or bug fixes.

## Credit
If you find this project to be useful in your academic work please cite the following paper:
[One, D. et al. Nature, 2021](https://nature.com)

The above README.md file will render on github as below.