Update docs/installing.md: add note about making sure of proper installation of required software

Until recently there have been a lot of questions about installing, which could have been avoided if required software such as compatible version of Ansible was installed. This note should encourage those who experience basic error to troubleshoot before giving up.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/installing.md

@@ -34,6 +34,8 @@ ansible-playbook -i inventory/hosts setup.yml --tags=install-all,ensure-matrix-u
 
 This will do a full installation and start all Matrix services.
 
+**Note**: if the command does not work as expected, make sure that you have properly installed and configured software required to run the playbook, as described on [Prerequisites](prerequisites.md).
+
 ### Installing a server into which you'll import old data
 
 If you will be importing data into your newly created Matrix server, install it, but **do not** start its services just yet. Starting its services or messing with its database now will affect your data import later on.