apt-key deprecation from Ubuntu 20.04 (#200)

Added note about apt-key deprecation.

Author: HofiOne

_includes/doc/admin-guide/notes/apt-key-deprecation.md

@@ -0,0 +1,2 @@
+**NOTE:** Note that `apt-key` authentication command line, has been deprecated in Ubuntu 20.04 and later versions.
+{: .notice--info}

doc/_dev-guide/chapter_0/section_1.md

@@ -11,6 +11,8 @@ id: dev-inst-debuan-ubuntu
 This chapter describes how you can install {{ site.product.short_name }} on Debian and on Ubuntu
 operating systems from our APT repository.
 
+{% include doc/admin-guide/notes/apt-key-deprecation.md %}
+
 ## syslog-ng
 
 ## Install {{ site.product.short_name }} from APT repository
@@ -81,6 +83,8 @@ Repositories are available [here][ref:obs-lbudai-36] and [here][ref:obs-lbudai-3
     wget -qO -  http://download.opensuse.org/repositories/home:/laszlo_budai:/syslog-ng/Debian_8.0/Release.key | sudo apt-key add -
     ```
 
+{% include doc/admin-guide/notes/apt-key-deprecation.md %}
+
 2. add repo to APT sources
 
     ```shell

doc/_dev-guide/chapter_5/section_1.md

@@ -81,6 +81,8 @@ Its documentation is extensive and full of examples. More test examples can be f
 
 ### Installation on Ubuntu
 
+{% include doc/admin-guide/notes/apt-key-deprecation.md %}
+
 ```
 $ curl http://download.opensuse.org/repositories/home:/laszlo_budai:/syslog-ng/xUbuntu_12.04/Release.key | sudo apt-key add -
 $ echo "deb http://download.opensuse.org/repositories/home:/laszlo_budai:/syslog-ng/xUbuntu_12.04 ./" | sudo tee --append /etc/apt/sources.list.d/syslog-ng-obs.list