Put the Github repo in maintenance mode (#2407)

Summary:
X-link: facebookresearch/fbpcp#533

X-link: facebookresearch/fbpcf#542

Pull Request resolved: #2407

According to our guideline (https://fb.workplace.com/groups/164332244998024/permalink/1234195941344977/), updating our open sourced repos with a sentence to indicate it's in maintenance mode.

Please let me know if the sentences are need to be rephrased.

Reviewed By: ajinkya-ghonge, joe1234wu, jinxinl22

Differential Revision: D58255798

fbshipit-source-id: 618e11d3d4cef52ae7373539863e3dd8fdc38776

README.md

@@ -1,3 +1,8 @@
+![Maintenance Mode](https://img.shields.io/badge/maintenance%20mode-%E2%9C%94%EF%B8%8F-red)
+**THIS PROJECT IS CURRENTLY IN MAINTENANCE MODE. Only critical bug patches will be applied; no new features will be added.**
+
+---
+
 # PCS (Private Computation Solutions)
 Private Computation Solutions provides multiple solutions that leverage various Privacy Enhancing Technologies (e.g., multi-party computation, differential privacy) used across Measurement and Signals. The existing solutions include Private Lift and Private Attribution, which allow calculations of Lift and Attribution metrics without revealing opportunity or conversion data on an individual level through 2PC.