graphql: Add -backend suffix to the plugin directory

plugins/graphql is a backend plugin published at @backstage/plugin-graphql-backend
When exploring the repository, I got confused to find only plugins/graphql since backend plugins are supposed to have the -backend suffix

Signed-off-by: Himanshu Mishra <himanshu@orkohunter.net>

plugins/graphql-backend/README.md

@@ -0,0 +1,28 @@
+# GraphQL Backend
+
+## Getting Started
+
+This is the GraphQL Backend plugin.
+
+It is responsible for merging different `graphql-plugins` together to provide the end schema.
+
+To run it within the backend do:
+
+1. Register the router in `packages/backend/src/index.ts`:
+
+```ts
+const graphqlEnv = useHotMemoize(module, () => createEnv('graphql'));
+
+const service = createServiceBuilder(module)
+  .loadConfig(configReader)
+  /** several different routers */
+  .addRouter('/graphql', await graphql(graphqlEnv));
+```
+
+2. Start the backend
+
+```bash
+yarn workspace example-backend start
+```
+
+This will launch the full example backend.