Add documentation for debian-9

[ci skip]

docs/installation/packaged/debian-9/README.md

@@ -0,0 +1,37 @@
+### Steps to install OpenProject on Debian 9 (Stretch)
+
+All steps need to be run as `root`.
+
+**1. Import the packager.io repository signing key**
+
+Import the PGP key used to sign our packages. Since we're using the _packager.io_ platform to distribute our packages, both package source and signing key are tied to their service.
+
+```bash
+wget -qO- https://dl.packager.io/srv/opf/openproject-ce/key | sudo apt-key add -
+```
+
+**2. Install apt-https suppport**
+
+Since we only provide https package sources, you may need to install `apt-transport-https` as a preliminary step.
+
+```bash
+apt-get install apt-transport-https
+```
+
+
+**3. Add the OpenProject package source**
+
+```
+wget -O /etc/apt/sources.list.d/openproject-ce.list \
+  https://dl.packager.io/srv/opf/openproject-ce/stable/7/installer/debian/9.repo
+```
+
+
+**4. Install the OpenProject Community Edition package**
+
+Using the following commands, apt will check the new package source and install the package and all required dependencies.
+
+```bash
+apt-get update
+apt-get install openproject
+```