Add info about emails into README

https://github.com/YunoHost-Apps/pyinventory_ynh/issues/53 works out of the box ;)

README.md

@@ -22,7 +22,35 @@ Pull requests welcome ;)
 ## Settings and upgrades
 
 Almost everything related to PyInventory's configuration is handled in a `"../conf/settings.py"` file.
-You can edit the file `$final_path/local_settings.py` to enable or disable features.
+You can edit the file `/opt/yunohost/pyinventory/local_settings.py` to enable or disable features.
+
+Test sending emails:
+
+```bash
+ssh admin@yourdomain.tld
+root@yunohost:~# cd /opt/yunohost/pyinventory/
+root@yunohost:/opt/yunohost/pyinventory# source venv/bin/activate
+(venv) root@yunohost:/opt/yunohost/pyinventory# ./manage.py sendtestemail --admins
+```
+
+Background info: Error mails are send to all [settings.ADMINS](https://docs.djangoproject.com/en/2.2/ref/settings/#std:setting-ADMINS). By default the YunoHost admin is inserted here.
+To check current ADMINS run:
+
+```bash
+(venv) root@yunohost:/opt/yunohost/pyinventory# ./manage.py sendtestemail --admins
+```
+
+If you prefere to send error emails to a extrnal email address, just do something like this:
+
+```bash
+echo "ADMINS = (('Your Name', 'example@domain.tld'),)" >> /opt/yunohost/pyinventory/local_settings.py
+```
+
+To check the effective settings, run this:
+```bash
+(venv) root@yunohost:/opt/yunohost/pyinventory# ./manage.py diffsettings
+```
+
 
 # Miscellaneous