README: add $schema for tooling assistance (#20)

This should make it easier for devs to author this file (at least until there's more tooling to generate/machine-maintain it)

readme.md

diff --git a/readme.md b/readme.md
index c71480f..4700779 100644
--- a/readme.md
+++ b/readme.md
@@ -10,6 +10,7 @@
 
 ```json
 {
+  "$schema": "https://raw.githubusercontent.com/theoremlp/rules_multitool/main/lockfile.schema.json",
   "tool-name": {
     "binaries": [
       {