Added information regarding environment variables

Installation.md

@@ -55,5 +55,31 @@ source .venv/bin/activate
 .venv/bin/pip --require-virtualenv install -r requirements.txt
 ```
 
+# Set environment variables
+
+Set the following environment variables:
+
+```sh
+export ZABBIX_HOST="https://zabbix.local"
+export ZABBIX_USER="username"
+export ZABBIX_PASS="Password"
+export NETBOX_HOST="https://netbox.local"
+export NETBOX_TOKEN="secrettoken"
+```
+
+Or, you can use a Zabbix API token to login instead of using a username and
+password. In that case `ZABBIX_USER` and `ZABBIX_PASS` will be ignored.
+
+```sh
+export ZABBIX_TOKEN=othersecrettoken
+```
+
+If you are using custom SSL certificates for NetBox and/or Zabbix, you can set
+the following environment variable to the path of your CA bundle file:
+
+```sh
+export REQUESTS_CA_BUNDLE=/path/to/your/ca-bundle.crt
+```
+
 # Configuration
 After installing the script you can start and run with the default settings. If you want to have more control over all of the features then this information is provided over at the [configuration](configuration) page.