Add glossary item for "pull" (#65)

Author: todaywasawesome

GLOSSARY.md

@@ -18,6 +18,13 @@ This glossary accompanies the [GitOps Principles](./PRINCIPLES.md), and other su
 - ## Drift
 
     When a system's actual state has moved or is in the process of moving away from the [desired state](#desired-state), this is often referred to as drift.
+    
+- ## Pull
+
+    [Principle 3](./PRINCIPLES.md) specifies the desired state must be "pulled" rather than "pushed", primarily because the software agents must be able to access the [desired state](#desired-state) from the [state store](#state-store) at _any_ time, not only when there is an intentional change in the state store triggering a push event.
+    This is a prerequisite for [reconciliation](#reconciliation) to happen [continuously](#continuous), as specified in [principle 4](./PRINCIPLES.md).
+    Note that – in contrast to traditional CI/CD, where automation is generally driven by pre-set triggers – in GitOps, [reconciliation](#reconciliation) is triggered _whenever_ there is a divergence.
+    Divergence could be due to the actual state unintentionally [drifting](#drift) from the desired state declarations – not only due to a new desired state declaration version having been changed intentionally.
 
 - ## Reconciliation
 

PRINCIPLES.md

@@ -15,7 +15,7 @@ The [desired state](./GLOSSARY.md#desired-state) of a GitOps managed system must
 
 - ## Pulled Automatically
 
-    Software agents automatically pull the desired state declarations from the source.
+    Software agents automatically [pull]((./GLOSSARY.md#pull)) the desired state declarations from the source.
 
 - ## Continuously Reconciled