Add maintenance notice to README (#852)

Note the non-maintained status of this project. Part of edgi-govdata-archiving/web-monitoring#168.

README.md

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 [![Code of Conduct](https://img.shields.io/badge/%E2%9D%A4-code%20of%20conduct-blue.svg?style=flat)](https://github.com/edgi-govdata-archiving/overview/blob/main/CONDUCT.md)  [![Project Status Board](https://img.shields.io/badge/✔-Project%20Status%20Board-green.svg?style=flat)](https://github.com/orgs/edgi-govdata-archiving/projects/4)
 
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+⚠️ **This project is no longer maintained.** ⚠️ It may receive security updates, but we are no longer making major changes or improvements. EDGI no longer makes active use of this toolset and it is hard to re-deploy in other contexts.
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 # web-monitoring-processing
 
 A component of the EDGI [Web Monitoring Project](https://github.com/edgi-govdata-archiving/web-monitoring).