docs: migrate to codeberg

wiki and release migration are TODO

README.md

@@ -7,10 +7,16 @@ Install from your [package manager](https://repology.org/project/river/versions)
 Join us at [#river](https://web.libera.chat/?channels=#river) on irc.libera.chat —
 Read our man pages and [wiki](https://github.com/riverwm/river/wiki)
 
+The main repository is on [codeberg](https://codeberg.org/river/river),
+which is where the issue tracker may be found and where contributions are accepted.
+
+Read-only mirrors exist on [sourcehut](https://git.sr.ht/~ifreund/river)
+and [github](https://github.com/riverwm/river).
+
 *Note: river is currently early in development. Expect breaking
 changes and missing features. Bugs should however be rare at
 this point, if you run into one don't hesitate to [open an
-issue](https://github.com/riverwm/river/issues/new)*
+issue](https://codeberg.org/river/river/issues/new)*
 
 ## Design goals