external_package.json: Add symbolic link to package.json

Since I7e313dd111 plugins/package.json is exposing typescript-api that
the plugin might depend on. Previously, the supported way to add custom
frontebd plugin dependencies was to overwrite the plugins/package.json
in gerrit core.

Bug: Issue 15931
Change-Id: I570a121e1af3801fc789b5eb73d95fc93be75920

README.md

diff --git a/README.md b/README.md
index cffe021..d29394d 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@
 ### Build
 
 1. Clone the package into `plugins` folder of `gerrit` repo or link the plugin folder to `gerrit/plugins` folder
-2. Override `plugins/package.json` with `package.json`, you can remove all `depDependencies` as they are not needed for the build (needed for running wct tests)
+2. Override `plugins/package.json` with `external_package.json`, you can remove all `depDependencies` as they are not needed for the build (needed for running wct tests)
 3. Run `bazel build plugins/image-diff`
 
 ## Highlight Mode

external_package.json

diff --git a/external_package.json b/external_package.json
new file mode 120000
index 0000000..2ff8622
--- /dev/null
+++ b/external_package.json
@@ -0,0 +1 @@
+package.json
\ No newline at end of file