Update README with workaround for entrypoint option

Until buildkite-plugins#138 is resolved (possibly by buildkite-plugins#132), this documentation
update should save users some pain when trying to override or
disable the default entrypoint for a container.

README.md

@@ -161,9 +161,9 @@ Default: `false`
 
 ### `entrypoint` (optional, string or boolean)
 
-Override the image’s default entrypoint, and defaults the `shell` option to `false`. See the [docker run --entrypoint documentation](https://docs.docker.com/engine/reference/run/#entrypoint-default-command-to-execute-at-runtime) for more details. Set it to `false` to disable the default entrypoint for the image.
+Override the image’s default entrypoint, and defaults the `shell` option to `false`. See the [docker run --entrypoint documentation](https://docs.docker.com/engine/reference/run/#entrypoint-default-command-to-execute-at-runtime) for more details. Set it to `""` (empty string) to disable the default entrypoint for the image (you may also need to use this plugin's `command` option instead of the top-level `command` option - see [Issue 138](https://github.com/buildkite-plugins/docker-buildkite-plugin/issues/138) for more information).
 
-Example: `/my/custom/entrypoint.sh`
+Example: `/my/custom/entrypoint.sh`, `""`
 
 ### `environment` (optional, array)