Added linear prediction how-to (librenms#10581)

As per twitter request.

doc/Support/FAQ.md

@@ -40,6 +40,7 @@ path: blob/master/doc/
 - [My reverse proxy is not working](#my-reverse-proxy-is-not-working)
 - [My alerts aren't being delivered on time](#my-alerts-aren't-being-delivered-on-time)
 - [My alert templates stopped working](#my-alert-templates-stopped-working)
+- [How do I use trend prediction in graphs](#how-do-i-use-trend-prediction-in-graphs)
 
 # Developing
 
@@ -573,4 +574,16 @@ new template syntax:
 syntax changed, and you basically need to convert your templates to
 this new syntax (including the titles).
 
+## <a name='how-do-i-use-trend-prediction-in-graphs'>How do I use trend prediction in graphs</a>
+
+As of [Ver. 1.55](https://community.librenms.org/t/v1-55-release-changelog-august-2019/9428) a new feature has been added where you can view a simple linear prediction in port graphs. 
+
+> It doesn't work on non-port graphs or consolidated graphs at the time this FAQ entry was written.
+
+To view a prediction:
+- Click on any `port` graph of any network device
+- Select a `From` date to your liking (not earlier than the device was actually added to LNMS), and then select a future date in the `To` field. 
+- Click update
+
+You should now see a linear prediction line on the graph.