Create challenge_7.md

Author: shivaylamba

Challenges/challenge_7.md

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+## Welcome to Challenge 7
+
+Welcome to the seventh challenge! 
+Today's goal is to implement the squashing of the git commits. Squashing is used to combine multiple commits into one. This is done using the interactive mode of Git Rebase command.
+In the [6th Challenge](https://github.com/scaleracademy/scaler-september-open-source-challenge/blob/main/Challenges/challenge_6.md?plain=1) all of you created an additional commit to update the markdown file. 
+So refer to [this guide](https://www.freecodecamp.org/news/git-squash-explained/) on Git merge. 
+
+Task: 
+1. Using the interactive mode of Git Rebase command combine the multiple commits created in your Pull requests and then push that to the Pull request you have created in the previous challenges.