INSTALL: add instructions for the Fedora copr

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

INSTALL.md

@@ -5,6 +5,22 @@
 Thanks to [atenart](https://ack.tf/), there is a package available on
 [Fedora's copr]( https://copr.fedorainfracloud.org/coprs/atenart/mat2/ ).
 
+We use copr (cool other packages repo) as the Mat2 Nautilus plugin depends on
+python3-nautilus, which isn't available yet in Fedora (but is distributed
+through this copr).
+
+First you need to enable Mat2's copr:
+
+```
+dnf -y copr enable atenart/mat2
+```
+
+Then you can install both the Mat2 command and Nautilus extension:
+
+```
+dnf -y install mat2 mat2-nautilus
+```
+
 ## Debian
 
 There is currently no package for Debian. If you want to help to make this