add permit docs

docs/OpenBrush/smart-contracts/PSP22/Extensions/permit.md

@@ -0,0 +1,57 @@
+---
+sidebar_position: 3
+title: PSP22 Permit
+---
+
+This extension allows the user to approve a third-party to spend some tokens on their behalf by ECDSA signature.
+
+This example shows how you can reuse the implementation of
+[PSP22](https://github.com/Brushfam/openbrush-contracts/tree/main/contracts/src/token/psp22) token with [PSP22Permit](https://github.com/Brushfam/openbrush-contracts/tree/main/contracts/src/token/psp22/extensions/permit.rs) extension.
+
+## How to use this extension
+
+First, you should implement basic version of [PSP22](../psp22.md).
+
+After you should add implementation of PSP22Permit and Nonces via `#[openbrush::implementation(...,PSP22Permit, Nonces,...)]` attribute.
+
+Also, add a storage field of type `nonces::Data` , which stores the number of nonces for each account, and a storage field of type `permit::Data`,
+which stores the permit data.
+
+## Final code
+
+```rust
+#![cfg_attr(not(feature = "std"), no_std, no_main)]
+
+pub use crate::my_psp22_permit::*;
+
+#[openbrush::implementation(PSP22, PSP22Permit, Nonces)]
+#[openbrush::contract]
+pub mod my_psp22_permit {
+ use openbrush::traits::Storage;
+
+ #[ink(storage)]
+ #[derive(Default, Storage)]
+ pub struct Contract {
+ #[storage_field]
+ psp22: psp22::Data,
+ #[storage_field]
+ nonces: nonces::Data,
+ #[storage_field]
+ psp22_permit: permit::Data,
+ }
+
+ impl Contract {
+ #[ink(constructor)]
+ pub fn new(total_supply: Balance) -> Self {
+ let mut instance = Self::default();
+
+ psp22::Internal::_mint_to(&mut instance, Self::env().caller(), total_supply).expect("Should mint");
+
+ instance
+ }
+ }
+}
+```
+
+And that's it! Your `PSP22` is now extended by the `PSP22Permit` extension and ready to use its functions!
+You can check an example of the usage of [PSP22 Permit](https://github.com/Brushfam/openbrush-contracts/tree/main/examples/psp22_extensions/permit).