docs: create CONTRIBUTING.md

Author: erictleung

CONTRIBUTING.md

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+# Contributing
+
+Hello and welcome! Here are some notes on how to contribute to this repository.
+
+## Prerequisites
+
+This repository aims to depend on as little dependencies as possible to make it simple.
+
+Currently, all that is needed is Python and it's standard library.
+
+## Files
+
+```
+.
+├── convert_json.py    # Converts JSON to RSS
+├── convert_readme.py  # Converts README into JSON
+├── emails.json        # Weekly emails in JSON form
+├── emails.rss         # Weekly emails in RSS form
+├── LICENSE
+└── README.md          # Weekly emails in Markdown form
+
+0 directories, 6 files
+```
+
+## Parsing files
+
+There are two Python scripts that help parse and transform the Markdown information:
+
+1. convert_json.py
+2. convert_readme.py