[v8.7.0] added change log for version 8.7.0 (#3878)

frappe/change_log/v8/v8_7_0.md

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+### User Permissions
+- User Permission is now a DocType, a new UX for the existing Role and User Permission managers to make it easy to enter permissions. For more details please check <a href="https://erpnext.org/docs/user/manual/en/setting-up/users-and-permissions/user-permissions">User Permissions</a>