Update the docs with a section regarding the cache usage

Signed-off-by: Soule BA <soule@weave.works>

docs/spec/v1beta2/helmcharts.md

@@ -624,3 +624,41 @@ reconcile](#triggering-a-reconcile).
 
 [typical-status-properties]: https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api-conventions.md#typical-status-properties
 [kstatus-spec]: https://github.com/kubernetes-sigs/cli-utils/tree/master/pkg/kstatus
+
+
+## Using The Helm Index
+
+When using a Helm Repository as source for a Helm chart and dependencies, the controller loads the Helm Repository index in-memory to find the latest version of the chart.
+
+The controller can be configured to cache a Helm Repository indexes in-memory. The cache is used to avoid loading Helm Repository indexes for every Helm chart reconciliation.
+
+Three fields are provided to enable and configure the cache:
+- `helm-cache-max-size`: The maximum size of the cache in number of items. If `0`, then the cache is disabled.
+- `helm-cache-ttl`: The TTL of an item in the cache.
+- `helm-cache-purge-interval`: The interval at which the cache is purged of expired items. 
+
+The caching strategy is to pull a Helm Repository index from the cache if it is available, otherwise to load the index, to retrieve and build the chart, then cache the index. The cached index ttl is refreshed every time the Helm Repository index is loaded with the `helm-cache-ttl` value.
+
+The cache is purged of expired items every `helm-cache-purge-interval`.
+
+When the cache is full, no more items can be added to the cache, the source-controller will report a warning event instead.
+
+In order to use the cache, set the related flags in the source-controller deployment config:
+
+```yaml
+    spec:
+      containers:
+      - args:
+        - --watch-all-namespaces
+        - --log-level=info
+        - --log-encoding=json
+        - --enable-leader-election
+        - --storage-path=/data
+        - --storage-adv-addr=source-controller.$(RUNTIME_NAMESPACE).svc.cluster.local.
+        ## Helm Cache with up to 10 items, i.e. 10 indexes.
+        - --helm-cache-max-size=10
+        ## TTL of an item is 1 hour.
+        - --helm-cache-ttl=1h
+        ## Purge expired items every 10 minutes.
+        - --helm-cache-purge-interval=10m
+```