Add an INSTALL.md file

INSTALL.md

+# GNU/Linux
+
+## Fedora
+
+Thanks to [atenart](https://ack.tf/), there is a package available on
+[Fedora's copr]( https://copr.fedorainfracloud.org/coprs/atenart/mat2/ ).
+
+## Debian
+
+There is currently no package for Debian. If you want to help to make this
+happen, there is an [issue](https://0xacab.org/jvoisin/mat2/issues/16) open.
+
+But fear not, there is a way to install it *manually*:
+
+```
+# apt install python3-mutagen python3-gi-cairo python3-gi-cairo gir1.2-gdkpixbuf-2.0 libimage-exiftool-perl gir1.2-glib-2.0 gir1.2-poppler-0.18
+$ git clone https://0xacab.org/jvoisin/mat2.git
+$ cd mat2
+$ ./mat2
+```
+
+and if you want to install the über-fancy Nautilus extension:
+
+```
+# apt install python-gi-dev
+$ git clone https://github.com/GNOME/nautilus-python
+$ cd nautilus-python
+$ PYTHON=/usr/bin/python3 ./autogen.sh
+$ make
+# make install
+$ cp ./nautilus/nautilus_mat2.py ~/.local/share/nautilus-python/extensions/
+$ PYTHONPATH=/home/$USER/mat2 PYTHON=/usr/bin/python3 nautilus
+```
+