Add start on filesync doc (GoogleContainerTools#1994)

docs/content/en/docs/how-tos/filesync/_index.md

@@ -6,4 +6,43 @@ weight: 40
 
 This page discusses how to set up file sync for files that don't require full rebuild.
 
+{{< alert title="Note" >}}
+File sync is alpha and may change between releases.
+{{< /alert >}}
+
+Skaffold supports copying changed files to a deployed containers so as to avoid the need to
+rebuild, redeploy, and restart the corresponding pod.  The file copying is enabled
+by adding a `sync` section with _sync rules_ to the `artifact` in the `skaffold.yaml`.
+
+The following example will cause any changes to JavaScript files under the _context_ directory
+to be copied to the deployed container into the container's `WORKDIR`.
+
+```yaml
+apiVersion: skaffold/v1beta8
+kind: Config
+build:
+  artifacts:
+  - image: gcr.io/k8s-skaffold/node-example
+    context: node
+    sync:
+      '.filebaserc': .
+      '*.html': static
+      '**/*.png': assets
+      '***/*.md': content
+```
+A double-asterisk (`**/`) applies recursively to all subdirectories but flattens the result,
+stripping the subdirectory structure.
+A triple-asterisk (`***/`) applies recursively to all subdirectories but retains
+the subdirectory structure.  
+
+Under the hood, Skaffold monitors and creates a tar file with changed files that match
+the sync rules.  This tar file is sent and extracted on the corresponding containers. 
+
+### Limitations
+
+File sync has some limitations:
+
+  - File sync requires the `tar` command to be available in the container.
+  - Only local source files can be synchronized: files created by the builder will not be copied.
+
 {{% todo 1076 %}}