Add explanation why different solc used #86

contracts/0.4.24/DePool.sol

@@ -1,3 +1,4 @@
+/* See contracts/COMPILERS.md */
 pragma solidity 0.4.24;
 
 import "@aragon/os/contracts/apps/AragonApp.sol";

contracts/0.4.24/StETH.sol

@@ -1,3 +1,4 @@
+/* See contracts/COMPILERS.md */
 pragma solidity 0.4.24;
 
 import "@aragon/os/contracts/apps/AragonApp.sol";

contracts/0.4.24/oracle/DePoolOracle.sol

@@ -1,3 +1,4 @@
+/* See contracts/COMPILERS.md */
 pragma solidity 0.4.24;
 
 import "@aragon/os/contracts/apps/AragonApp.sol";

contracts/0.4.24/sps/StakingProvidersRegistry.sol

@@ -1,3 +1,4 @@
+/* See contracts/COMPILERS.md */
 pragma solidity 0.4.24;
 
 import "@aragon/os/contracts/apps/AragonApp.sol";

contracts/0.6.12/CstETH.sol

@@ -1,4 +1,5 @@
-pragma solidity 0.6.12; // latest available for using OZ
+/* See contracts/COMPILERS.md */
+pragma solidity 0.6.12;
 
 import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 import "@openzeppelin/contracts/token/ERC20/ERC20Burnable.sol";

contracts/COMPILERS.md

@@ -0,0 +1,20 @@
+# Why we use different compilers
+
+For DePool project coordination, governance and funds management we use [Aragon](https://aragon.org/dao), a well-developed and proven DAO Framework. The current stable release of its Kernel, [4.4.0](https://github.com/aragon/aragonOS/tree/v4.4.0) is fixed on the specific compiler version - [solc 0.4.24](https://solidity.readthedocs.io/en/v0.4.24/), that is currently outdated. Keeping security and consistency in mind, we decided to stay on an older yet proven combination - for all the contracts under Aragon management (`DePool`, `stETH`, `DePoolOracle`) we use solc 0.4.24 release.
+
+cstETH token, that acts as autonomous wrapper and not governed by Aragon, was inherited from OpenZeppelin's library, using one of its stable releases [3.1.0](https://github.com/OpenZeppelin/openzeppelin-contracts/releases/tag/v3.1.0).
+
+# How to compile
+
+Separately:
+
+```bash
+yarn compile:4
+yarn compile:6
+```
+
+All at once:
+
+```bash
+yarn compile
+```