Added CONTRIBUTING.md

Added CONTRIBUTING.md file to the repo

CONTRIBUTING.md

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+# Contributing to awesome-sre-tools
+
+Anyone can contribute to this list, just make sure you follow the below guidelines.
+This guide may evolve from time to time, please make sure you adhere to these guidelines whenever you create a PR.
+
+- No duplicate entries.
+- Add your listing at the bottom of each section.
+- Make sure there are no dead url's in your submission.
+- If required, propose a new section and add it to the Table of contents
+- Provide proper commit messages and mention what exactly you are doing in your PR (No marketing lingo, just the actual context)
+- If there is a merge conflict, please rebase and resolve the conflicts and then submit the PR again.