📝 Document auth decorators usage

Author: Pl217

docs/AUTH.md

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+# Authentication
+
+Authentication can be used as a guard on a field, query or mutation, restricting data access or actions for a specific group of users.
+
+Since the codebase uses TypeGraphQL, which relies heavily on decorators, authentication is also done using decorators.
+
+Authentication is done with use of `@Permission` decorator. This decorator takes function as an argument with permission object as a return value.
+
+For example:
+
+```lang=js
+@Permission(({ args }) =>
+  Promise.resolve({
+    or: [
+      { type: 'global', permission: P.global.VIEW_ANY_PLAN_DATA },
+      { type: 'plan', permission: P.plan.VIEW_DATA, id: args.id },
+    ],
+  })
+)
+```
+
+If only global permission check is needed, it can be used directly:
+
+```lang=js
+@Permission({
+  type: 'global',
+  permission: P.global.VIEW_ALL_JOBS,
+})
+```