update documentation

docs/lib/metatx/ERC2771Context.md

@@ -0,0 +1,53 @@
+# Solidity API
+
+## Context
+
+_Provides information about the current execution context, including the
+sender of the transaction and its data. While these are generally available
+via msg.sender and msg.data, they should not be accessed in such a direct
+manner, since when dealing with meta-transactions the account sending and
+paying for execution may not be the actual sender (as far as an application
+is concerned).
+
+This contract is only required for intermediate, library-like contracts._
+
+### _msgSender
+
+```solidity
+function _msgSender() internal view virtual returns (address)
+```
+
+### _msgData
+
+```solidity
+function _msgData() internal view virtual returns (bytes)
+```
+
+## ERC2771Context
+
+_Context variant with ERC2771 support._
+
+### trustedForwarders
+
+```solidity
+mapping(address => bool) trustedForwarders
+```
+
+### _msgSender
+
+```solidity
+function _msgSender() internal view virtual returns (address sender)
+```
+
+### isTrustedForwarder
+
+```solidity
+function isTrustedForwarder(address forwarder) public view virtual returns (bool)
+```
+
+### _msgData
+
+```solidity
+function _msgData() internal view virtual returns (bytes)
+```
+