ERC DelegateProxy (#897)

* Add ERCProxy draft

* Add missing links

* Re-format EIP897

* ERC897: add reference to implementations

EIPS/eip-897.md

@@ -0,0 +1,73 @@
+---
+eip: 897
+title: ERC DelegateProxy
+author: Jorge Izquierdo <jorge@aragon.one>, Manuel Araoz <manuel@zeppelin.solutions>
+type: Standards Track
+category: ERC
+status: Draft
+created: 2018-02-21
+discussions-to: https://github.com/ethereum/EIPs/pulls/897
+---
+
+## Simple Summary
+Proxy contracts are being increasingly used as both as an upgradeability mechanism
+and a way to save gas when deploying many instances of a particular contract. This
+standard proposes a set of interfaces for proxies to signal how they work and what
+their main implementation is.
+
+## Abstract
+Using proxies that delegate their own logic to another contract is becoming an
+increasingly popular technique for both smart contract upgradeability and creating
+cheap clone contracts.
+
+We don't believe there is value in standardizing any particular implementation
+of a DelegateProxy, given its simplicity, but we believe there is a lot of value
+in agreeing on an interface all proxies use that allows for a standard way to
+operate with proxies.
+
+## Implementations
+
+- **aragonOS**: [AppProxyUpgradeable](https://github.com/aragon/aragonOS/blob/dev/contracts/apps/AppProxyUpgradeable.sol), [AppProxyPinned](https://github.com/aragon/aragonOS/blob/dev/contracts/apps/AppProxyPinned.sol) and [KernelProxy](https://github.com/aragon/aragonOS/blob/dev/contracts/kernel/KernelProxy.sol)
+
+- **zeppelinOS**: [Proxy](https://github.com/zeppelinos/labs/blob/2da9e859db81a61f2449d188e7193788ca721c65/upgradeability_ownership/contracts/Proxy.sol)
+
+## Standardized interface
+
+```solidity
+interface ERCProxy {
+  function proxyType() public pure returns (uint256 proxyTypeId);
+  function implementation() public view returns (address codeAddr);
+}
+```
+
+### Code address (`implementation()`)
+The returned code address is the address the proxy would delegate calls to at that
+moment in time, for that message.
+
+### Proxy Type (`proxyType()`)
+
+Checking the proxy type is the way to check whether a contract is a proxy at all.
+When a contract fails to return to this method or it returns 0, it can be assumed
+that the contract is not a proxy.
+
+It also allows for communicating a bit more of information about how the proxy
+operates. It is a pure function, therefore making it effectively constant as
+it cannot return a different value depending on state changes.
+
+- **Forwarding proxy** (`id = 1`): The proxy will always forward to the same code
+address. The following invariant should always be true: once the proxy returns
+a non-zero code address, that code address should never change.
+
+- **Upgradeable proxy** (`id = 2`): The proxy code address can be changed depending
+on some arbitrary logic implemented either at the proxy level or in its forwarded
+logic.
+
+## Benefits
+
+- **Source code verification**: right now when checking the code of a proxy in explorers
+like Etherscan, it just shows the code in the proxy itself but not the actual
+code of the contract. By standardizing this construct, they will be able to show
+both the actual ABI and code for the contract.
+
+## Copyright
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).