adding documentation for base image caching (#421)

* adding documentation for base image caching

* add --cache-repo to list of params

README.md

@@ -210,6 +210,8 @@ We can run the kaniko executor image locally in a Docker daemon to build and pus
  ```
 
 ### Caching
+
+#### Caching Layers
 kaniko currently can cache layers created by `RUN` commands in a remote repository. 
 Before executing a command, kaniko checks the cache for the layer.
 If it exists, kaniko will pull and extract the cached layer instead of executing the command.
@@ -219,6 +221,21 @@ Users can opt in to caching by setting the `--cache=true` flag.
 A remote repository for storing cached layers can be provided via the `--cache-repo` flag.
 If this flag isn't provided, a cached repo will be inferred from the `--destination` provided. 
 
+#### Caching Base Images
+kaniko can cache images in a local directory that can be volume mounted into the kaniko image. 
+To do so, the cache must first be populated, as it is read-only. We provide a kaniko cache warming 
+image at `gcr.io/kaniko-project/warmer`:
+
+```shell
+docker run -v $(pwd):/workspace gcr.io/kaniko-project/warmer:latest --cache-dir=/workspace/cache --image=<image to cache> --image=<another image to cache>
+```
+
+`--image` can be specified for any number of desired images. 
+This command will cache those images by digest in a local directory named `cache`.
+Once the cache is populated, caching is opted into with the same `--cache=true` flag as above. 
+The location of the local cache is provided via the `--cache-dir` flag, defaulting at `/cache` as with the cache warmer. 
+See the `examples` directory for how to use with kubernetes clusters and persistent cache volumes.
+
 ### Pushing to Different Registries
 
 kaniko uses Docker credential helpers to push images to a registry.
@@ -345,6 +362,12 @@ If `--destination=gcr.io/kaniko-project/test`, then cached layers will be stored
 
 _This flag must be used in conjunction with the `--cache=true` flag._
 
+#### --cache-dir
+
+Set this flag to specify a local directory cache for base images. Defaults to `/cache`.
+
+_This flag must be used in conjunction with the `--cache=true` flag._
+
 #### --cleanup
 
 Set this flag to cleanup the filesystem at the end, leaving a clean kaniko container (if you want to build multiple images in the same container, using the debug kaniko image)