[gtk-web: 1/5] Add package manager info to install page

[Emmanuele Bassi <ebassi src gnome org>]

commit ea919a9d16ce794a35896d73aeda0c82d3a0a347
Author: Olivier FAURE <couteaubleu gmail com>
Date:   Mon Oct 19 19:49:32 2020 +0000

    Add package manager info to install page
    
    Finding the Debian package for gtk3 in particular is a bit of a pain. For instance, searching "gtk" or 
"gtk3" in https://packages.ubuntu.com/ doesn't yield the correct package.
    
    Since this page was the first hit I searched "installing gtk3", I'm proposing to add basic instructions 
to find the correct package here.

 _docs/installations/linux.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
---
diff --git a/_docs/installations/linux.md b/_docs/installations/linux.md
index 323892d6..5bc7c972 100644
--- a/_docs/installations/linux.md
+++ b/_docs/installations/linux.md
@@ -26,10 +26,24 @@ Epoxy | [<i class="fas fa-download"></i>](https://download.gnome.org/sources/lib
 
 The current stable API version of GTK is 4.0.
 
+### Building from source
+
 To build GTK, see the [installation guide](https://developer.gnome.org/gtk4/unstable/gtk-building.html).
 For additional help, the [frequently asked questions 
page](https://developer.gnome.org/gtk4/unstable/gtk-question-index.html) is a
 good starting point.
 
+### Debian / Ubuntu
+
+```
+sudo apt install libgtk-3-dev
+```
+
+### Arch
+
+```
+sudo pacman -S gtk3
+```
+
 ## Development version
 
 To build the latest development version of GTK you can use a tool like