Explain the Sentinel interface in more depth

Author: axic

sentinel.md

@@ -8,3 +8,27 @@ The process of metering is [explained in its own chapter](./metering.md).
 
 The Sentinel is a system contract which means its code is fixed and is part of the genesis or hardfork ewasm is enabled on.
 As a result, changing or updating it requires a hard fork.
+
+The Sentinel contract has two interfaces:
+- raw interface
+- ABI encoded interface
+
+## Error handling
+
+In both modes, the contract will use an invalid instruction (e.g. regular failure) when an unexpected issue occurs,
+or the `revert` operation if the supplied input is invalid.
+
+## Raw interface
+
+In the raw interface it accepts any valid WebAssembly binary, applies the Sentinel steps, and returns the transformed
+WebAssembly binary. There is no encapsulation of any kind.
+
+## ABI encoded interface
+
+The ABI encoded interface is demonstrated by the following Solidity interface:
+```solidity
+interface Sentinel {
+     // Expects a WebAssembly binary and returns a transformed WebAssembly binary.
+     function validateAndMeter(bytes) external view returns (bytes);
+}
+```