ci skip: Update README.md

README.md

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 ![main](https://github.com/flowerinthenight/spindle/workflows/main/badge.svg)
 [![Go Reference](https://pkg.go.dev/badge/github.com/flowerinthenight/spindle.svg)](https://pkg.go.dev/github.com/flowerinthenight/spindle)
 
+(This repo is mirrored to [https://codeberg.org/flowerinthenight/zgroup](https://codeberg.org/flowerinthenight/spindle)).
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 ## spindle
 A distributed locking library built on top of [Cloud Spanner](https://cloud.google.com/spanner/). It relies on Spanner's [TrueTime](https://cloud.google.com/spanner/docs/true-time-external-consistency) and [transactions](https://cloud.google.com/spanner/docs/transactions) support to achieve its locking mechanism.