Add Contributing Guide (CONTRIBUTING.md)

[sidntrivedi012]

First-time Contributors

If it is the first time that you contribute to the bot, follow these steps:

1. You need to have git & golang available on your machine.
   • To install Git - https://git-scm.com/book/en/v2/Getting-Started-Installing-Git
   • To setup golang, follow the doc here - https://golang.org/doc/install
2. Write a comment in this issue thread to let other possible contributors know that you are working on this bug. For eg : Hey all, I would like to work on this issue.
3. Ping Botfather on Telegram and make your instance of OSDC-Bot bot by selecting /newbot from the options it provides.
4. Copy the TELEGRAM_TOKEN provided by Botfather.
5. Fork this repo.
6. Run git clone https://github.com/<YOUR_USERNAME>/bots.git && cd bots/telegram-bot
7. If you have installed golang, run go build .
8. Wait ⏳
9. Run export TELEGRAM_TOKEN=<botfather_token>
10. Now, run ./telegram-bot. The bot would be running at the user handle provided by you.
11. Make changes according to the issue. Test the working of the changes.
12. Commit the changes and then run git push origin master and then open a PR! 🎉