Create challenge_12.md

Author: shivaylamba

Challenges/challenge_12.md

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+## Welcome to Challenge 12
+
+Welcome to the 12th Challenge 
+
+Today's challenge is a re-practice of the 7th Challenge (https://github.com/scaleracademy/scaler-september-open-source-challenge/blob/main/Challenges/challenge_7.md?plain=1)
+
+So focus for this challenge will be to practice the ``git rebase`` command. We still see a number of Pull requests that have a lot more commits than there should be, therefore we give you a chance to correct it again. 
+
+Ensure to practice the ``git rebase`` command by creating your own local git repository. You can checkout this guide: https://www.freecodecamp.org/news/git-squash-explained/
+
+Task: 
+1. Practice git rebase, and how to squash commit susing git rebase in your local projects 
+2. Once done, try to squash the commits in your pull request in the Challenge repository
+3. We also see a lot of challengers are not signing their commits. Refer to Challenge 10: https://github.com/scaleracademy/scaler-september-open-source-challenge/blob/main/Challenges/challenge_10.md and ensure to sign your commits.