Create README for IDPS Threat Category detections

Added a README.md detailing IDPS Threat Category based detections and contribution guidelines.

Author: shabaz-github

Azure Firewall/Alerts - Queries and Alerts/Detection - AZFW IDPS Threat Category based Detections/README.md

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+## IDPS Threat Category based detections
+
+1. High severity malicious activity detected
+2. Medium severity malicious activity detected
+3. Web Application attack detected
+4. DDoS attack detected
+5. Elevation of Privilege attempt detected
+
+
+
+## Contributing
+
+This project welcomes contributions and suggestions.  Most contributions require you to agree to a
+Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us
+the rights to use your contribution. For details, visit https://cla.opensource.microsoft.com.
+
+When you submit a pull request, a CLA bot will automatically determine whether you need to provide
+a CLA and decorate the PR appropriately (e.g., status check, comment). Simply follow the instructions
+provided by the bot. You will only need to do this once across all repos using our CLA.
+
+This project has adopted the [Microsoft Open Source Code of Conduct](https://opensource.microsoft.com/codeofconduct/).
+For more information see the [Code of Conduct FAQ](https://opensource.microsoft.com/codeofconduct/faq/) or
+contact [opencode@microsoft.com](mailto:opencode@microsoft.com) with any additional questions or comments.