feat: add semi modular account support wording (#161)

standard/ERCs/erc-6900.md

@@ -568,6 +568,12 @@ The `executeFromModule` function MUST allow modules to call execution functions
 
 The `executeFromModuleExternal` function MUST allow modules to call external addresses as specified by its parameters on behalf of the modular account. If the calling module's manifest did not explicitly allow the external call within `permittedExternalCalls` at the time of installation, execution MUST revert.
 
+### Extension
+
+#### Semi-modular account
+
+Account implementers may opt to design a semi-modular account, where certain features, such as default validation, are integrated into the core account. This approach should ensure compatibility with fully modular accounts, maintaining interoperability across different implementations.
+
 ## Rationale
 
 ERC-4337 compatible accounts must implement the `IAccount` interface, which consists of only one method that bundles validation with execution: `validateUserOp`. A primary design rationale for this proposal is to extend the possible functions for a smart contract account beyond this single method by unbundling these and other functions, while retaining the benefits of account abstraction.