Fix cross-reference to readme installation chapter

in CONTRIBUTING.md while keeping it functional in both .md and
Sphinx-included versions.

CONTRIBUTING.md

@@ -27,10 +27,13 @@ No matter if you're a programmer or not there are many ways to contribute to the
 
 ### Non-Programming
 
-* [Set up synadm](README.md#installation) and report whether the installation, configuration and update processes work as described or could be improved. We don't have the resources to test on many operating systems, but you can [let us know](https://matrix.to/#/%23synadm%3Apeek-a-boo.at?via=jacksonchen666.com&via=maclemon.at&via=matrix.org&via=raim.ist) if it does work.
+* [Set up synadm][readme-installation] and report whether the installation, configuration and update processes work as described or could be improved. We don't have the resources to test on many operating systems, but you can [let us know](https://matrix.to/#/%23synadm%3Apeek-a-boo.at?via=jacksonchen666.com&via=maclemon.at&via=matrix.org&via=raim.ist) if it does work.
 * Help keep the features list on [API to CLI Mapping](https://synadm.readthedocs.io/en/latest/features.html) up to date.
 * Improve the docs: The end-user documentation is entirely realized with the typical online help of Unix commands, an option named`--help/-h`. We believe that a top-priority in designing CLI tools is getting this information right. Spelling, wording and of course technical correctness are equally important. If it's not easily possible to stay brief and technically precise at the same time, we believe it is OK to prioritize precision over shortness. We also have documentation made with _Sphinx_ [available online](https://synadm.readthedocs.io/en/latest/index_cli_reference.html).
 
+
+[readme-installation]:https://github.com/JOJ0/synadm/blob/master/README.md#installation
+
 ### Programming
 
 * Pick an [open issue on GitHub](https://github.com/JOJ0/synadm/issues) or a not yet implemented [Synapse Admin API](https://synadm.readthedocs.io/en/latest/features.html) (see [API to CLI Mapping](https://synadm.readthedocs.io/en/latest/features.html)) and start implementing it, then submit a pull-request when you are ready.