README.md: refer to the new upstream location

kdudka · kdudka · commit 59368e8e8552 · 2021-08-06T16:56:15.000+02:00
diff --git a/README.md b/README.md
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+The upstream repository of `csdiff` has been moved to: https://github.com/csutils/csdiff

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	`1`	+The upstream repository of `csdiff` has been moved to: https://github.com/csutils/csdiff