Add FAQ for admin section (#2427)

See #1987 and #2102

docs/en/admins/01_Index.md

@@ -6,3 +6,4 @@ Learn how to install, update and backup FreshRSS and how to use the command line
 * [Update your installation](03_Updating.md) to the latest stable or dev version
 * [The command line interface](https://github.com/FreshRSS/FreshRSS/tree/master/cli) can be used to administrate feeds and users
 * [Automatic feed updates](https://github.com/FreshRSS/FreshRSS#automatic-feed-update) using cron is the preferred way to get the latest feeds entries   
+* [Frequently asked questions](04_Frequently_Asked_Questions.md)

docs/en/admins/04_Frequently_Asked_Questions.md

@@ -0,0 +1,28 @@
+We may not have answered all of your questions in the previous sections. The FAQ contains some questions that have not been answered elsewhere.
+
+## Promoting a user to admin
+
+At the moment, there can be only one *admin* user for the system.
+Thus promoting one user to *admin* demotes the current *admin* user.
+
+The recommended way of promoting a user is with the help of the CLI tool.
+You only have to do is to run the following command:
+```sh
+./cli/reconfigure.php --default_user <username>
+```
+
+Alternatively, you can edit configuration files manually.
+To do so, you need to change the *default_user* value in the file *./data/config.php*.
+As the file is a PHP file, you have to make sure that it's still valid after the update by running the following command:
+```sh
+php -l ./data/config.php
+```
+
+## Disabling self-registration
+
+Users can register directly on the login screen only if the configuration allows them.
+Under *Administration* > *System configuration*, you have access to *Max number of accounts*.
+As stated on that page, there is no limitation if you input **0**, thus allowing any number of user to self-register.
+If you input any other number, you will create a limitation on self-registering users.
+That means that as soon as the limit is reached, users cannot self-register but they can still be registered by the *admin* user.
+Using the value **1**, disables the self-registration since the spot is used by the *admin* user.