Instructions on installing RC debs (#10396)

Author: richvdh

changelog.d/10396.doc

@@ -0,0 +1 @@
+Add instructructions on installing Debian packages for release candidates.

docs/setup/installation.md

@@ -268,9 +268,8 @@ For more details, see
 
 ##### Matrix.org packages
 
-Matrix.org provides Debian/Ubuntu packages of the latest stable version of
-Synapse via <https://packages.matrix.org/debian/>. They are available for Debian
-9 (Stretch), Ubuntu 16.04 (Xenial), and later. To use them:
+Matrix.org provides Debian/Ubuntu packages of Synapse via
+<https://packages.matrix.org/debian/>.  To install the latest release:
 
 ```sh
 sudo apt install -y lsb-release wget apt-transport-https
@@ -281,12 +280,16 @@ sudo apt update
 sudo apt install matrix-synapse-py3
 ```
 
-**Note**: if you followed a previous version of these instructions which
-recommended using `apt-key add` to add an old key from
-`https://matrix.org/packages/debian/`, you should note that this key has been
-revoked. You should remove the old key with `sudo apt-key remove
-C35EB17E1EAE708E6603A9B3AD0592FE47F0DF61`, and follow the above instructions to
-update your configuration.
+Packages are also published for release candidates. To enable the prerelease
+channel, add `prerelease` to the `sources.list` line. For example:
+
+```sh
+sudo wget -O /usr/share/keyrings/matrix-org-archive-keyring.gpg https://packages.matrix.org/debian/matrix-org-archive-keyring.gpg
+echo "deb [signed-by=/usr/share/keyrings/matrix-org-archive-keyring.gpg] https://packages.matrix.org/debian/ $(lsb_release -cs) main prerelease" |
+    sudo tee /etc/apt/sources.list.d/matrix-org.list
+sudo apt update
+sudo apt install matrix-synapse-py3
+```
 
 The fingerprint of the repository signing key (as shown by `gpg
 /usr/share/keyrings/matrix-org-archive-keyring.gpg`) is