Update adv deployment doc (#809)

docs/advanced-function-deployment.md

@@ -25,13 +25,23 @@ The fields that a Function specification can contain are:
  - Runtime: Runtime ID and version that the function will use. It should match one of the availables in the [Kubeless configuration](/docs/function-controller-configuration).
  - Timeout: Maximum timeout for the given function. After that time, the function execution will be terminated.
  - Handler: Pair of `<file_name>.<function_name>`. When using `zip` in `function-content-type` the `<file_name>` will be used to find the file with the function to expose. In other case it will be used just as a final file name. `<function_name>` is used to select the function to run from the exported functions of `<file_name>`. This field is mandatory and should match with an exported function.
- - Deps: Dependencies of the function. The format of this field will depend on the runtime, e.g. a `package.json` for NodeJS functions or a `Gemfile` for Ruby. This field supports the use of URLs to specify dependency files.
+ - Deps: Dependencies of the function. The format of this field will depend on the runtime, e.g. a `package.json` for NodeJS functions or a `Gemfile` for Ruby.
  - Checksum: SHA256 of the function content.
  - Function content type: Content type of the function. Current supported values are `base64`, `url` or `text`. If the content is zipped the suffix `+zip` should be added.
  - Function: Function content.
 
 Apart from the basic parameters, it is possible to add the specification of a `Deployment`, a `Service` or an `Horizontal Pod Autoscaler` that Kubeless will use to generate them.
 
+## Deploying large functions
+
+As any Kubernetes object, function objects have a maximum size of 1MB (due to the maximum size of an etcd entry). Because of that, it's not possible to specify in the `function` field of the YAML content that surpasses that size. To workaround this issue it's possible to specify an URL in the `function` field. This file will be downloaded at build time (extracted if necessary) and the checksum will be checked. Doing this we avoid any limitation regarding the file size. It's also possible to include the function dependencies in this file and skip the dependency installation step. Note that since the file will be downloaded in a pod the URL should be accessible from within the cluster:
+
+```yaml
+  checksum: sha256:d1f84e9f0a8ce27e7d9ce6f457126a8f92e957e5109312e7996373f658015547
+  function: https://github.com/kubeless/kubeless/blob/master/examples/nodejs/helloFunctions.zip?raw=true
+  function-content-type: url+zip
+```
+
 ## Custom Deployment
 
 It is possible to specify a [`Deployment` spec](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#creating-a-deployment) in the Function spec that will be merged with default values set by the Kubeless controller. It is not necessary to specify all the fields of the deployment, just the fields you are interested on overwriting. For example: