add example of custom operator

Signed-off-by: goenning <me@goenning.net>

.changeset/spicy-moons-poke.md

@@ -1,5 +1,5 @@
 ---
-'@backstage/plugin-tech-insights-backend-module-jsonfc': minor
+'@backstage/plugin-tech-insights-backend-module-jsonfc': patch
 ---
 
 ability to add custom operators

plugins/tech-insights-backend-module-jsonfc/README.md

@@ -85,3 +85,32 @@ export const exampleCheck: TechInsightJsonRuleCheck = {
   },
 };
 ```
+
+# Custom operators
+
+json-rules-engine supports a limited [number of built-in operators](https://github.com/CacheControl/json-rules-engine/blob/master/docs/rules.md#operators) that can be used in conditions. You can add your own operators by adding them to the `operators` array in the `JsonRulesEngineFactCheckerFactory` constructor. For example:
+
+```diff
+const myFactCheckerFactory = new JsonRulesEngineFactCheckerFactory({
+   checks: [],
+   logger,
++  operators: [ new Operator("startsWith", (a, b) => a.startsWith(b) ]
+})
+```
+
+And you can then use it in your checks like this:
+
+```js
+...
+rule: {
+  conditions: {
+    any: [
+      {
+        fact: 'version',
+        operator: 'startsWith',
+        value: '12',
+      },
+    ],
+  },
+}
+```