Update Readme with more common issues

README.md

@@ -223,3 +223,7 @@ In most cases enabling debug logs will help to identify the issue. To enable deb
 * Check the logs to see if there are any timeouts or errors while scraping for metrics. In a large Artifactory instance, it may take a long time to scrape for all metrics especially `artifactory_artifacts_*` metrics. If there are any errors, try increasing the default timeout(5s) using `--artifactory.timeout` flag.
 * Some metrics are not available based on your version or license type. Check the [metrics](#metrics) section to see if the metric is available for your license type.
 * Some metrics are optional and are disabled by default. Check the [optional metrics](#optional-metrics) section to see available optional metrics. You can enable them using `--optional-metric=metric_name` flag. You can pass this flag multiple times to enable multiple optional metrics.
+
+#### There was an error when trying to unmarshal the API Error
+
+This error is usually means that the exporter is not able to properly reach the Artifactory endpoint. One of the common reasons for this is that the Artifactory URL is not set properly. Make sure your are not missing `/artifactory` at the end of the URL which is how most implementations of Artifactory are configured. (e.g. `http://artifactory.yourdomain.com/artifactory`)