Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC20/extensions/ERC20Burnable.sol

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.5.0) (token/ERC20/extensions/ERC20Burnable.sol)
+
+pragma solidity ^0.8.0;
+
+import "../ERC20.sol";
+import "../../../utils/Context.sol";
+
+/**
+ * @dev Extension of {ERC20} that allows token holders to destroy both their own
+ * tokens and those that they have an allowance for, in a way that can be
+ * recognized off-chain (via event analysis).
+ */
+abstract contract ERC20Burnable is Context, ERC20 {
+    /**
+     * @dev Destroys `amount` tokens from the caller.
+     *
+     * See {ERC20-_burn}.
+     */
+    function burn(uint256 amount) public virtual {
+        _burn(_msgSender(), amount);
+    }
+
+    /**
+     * @dev Destroys `amount` tokens from `account`, deducting from the caller's
+     * allowance.
+     *
+     * See {ERC20-_burn} and {ERC20-allowance}.
+     *
+     * Requirements:
+     *
+     * - the caller must have allowance for ``accounts``'s tokens of at least
+     * `amount`.
+     */
+    function burnFrom(address account, uint256 amount) public virtual {
+        _spendAllowance(account, _msgSender(), amount);
+        _burn(account, amount);
+    }
+}