docs: add documentation for the --destination flag

The flag is mostly self-explanatory, but its usage for multiple push
was not documented anywhere.

Related to #217, #733

README.md

@@ -78,6 +78,7 @@ _If you are interested in contributing to kaniko, see
       - [Flag `--compressed-caching`](#flag---compressed-caching)
       - [Flag `--context-sub-path`](#flag---context-sub-path)
       - [Flag `--custom-platform`](#flag---custom-platform)
+      - [Flag `--destination`](#flag---destination)
       - [Flag `--digest-file`](#flag---digest-file)
       - [Flag `--dockerfile`](#flag---dockerfile)
       - [Flag `--force`](#flag---force)
@@ -876,6 +877,16 @@ _This is not virtualization and cannot help to build an architecture not
 natively supported by the build host. This is used to build i386 on an amd64
 Host for example, or arm32 on an arm64 host._
 
+#### Flag `--destination`
+
+Path where to push the built image. You can set it multiple
+times for multiple registries, image names or tags.
+
+Example value: `gcr.io/my-repo/my-image:my-tag`
+
+When setting the destination to multiple image names, set the
+[`--cache-repo` flag](#flag---cache-repo) explicitly.
+
 #### Flag `--digest-file`
 
 Set this flag to specify a file in the container. This file will receive the