Add main.py, requirements.txt and README.md for Mail Merge

[gh-adithya]

Fix to Issue #133

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Added a Mail Merge project that auto-generates letters using a template, and with names provided.

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Checklist

• [x] I read the contributing guidelines
• [x] I structured my project as stated in the contributing guidelines
• [x] My project uses dependencies
• [x] I starred the repo
• [x] I had fun contributing!

[theadeyemiolayinka]

Project reviewed and merged. Thank you @gh-adithya Feel free to add more projects.