Add custom improvement proposal doc

docs/develop/smart-contracts/custom-improvement-proposals.md

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+# Custom Improvement Proposals
+
+With the release [v19.0.0](https://github.com/evmos/evmos/releases/tag/v19.0.0)
+a new feature called custom improvement proposals has been introduced in the
+evmOS framework. Custom improvement proposals allow protocol developers to
+modify the behavior of the EVM opcodes to tailor their functionalities to the
+specific needs.
+
+## Operations
+
+Operations are the base components of the Ethereum Virtual Machine (EVM) which
+allow the execution of the smart contract logic. When a developer builds a smart
+contract, the code written in Solidity, or Vyper, is not directly interpretable
+by the EVM. Before being able to execute the code in the blockchain, the
+contract has to be compiled via one of the available compilers, like
+[solc](https://docs.soliditylang.org/en/latest/using-the-compiler.html). The
+compilation converts the human-readable contract code into a sequence of operations
+that the virtual machine can interpret and execute
+to perform state transitions or query the latest committed state.
+These operations are called **opcodes**, and are contained
+in a structure called [**jump table**](https://github.com/evmos/evmos/blob/v19.0.0/x/evm/core/vm/jump_table.go#L120-L1094).
+
+Each opcode is defined by specifying the logic that has to be executed when it
+is called inside the EVM, its relationship with the memory, and the gas cost
+associated with it. More specifically, an opcode is completely defined by:
+
+- `SetExecute`: update the execution logic for the opcode.
+
+- `SetConstantGas`: update the value used for the constant gas cost.
+
+- `SetDynamicGas`: update the function used to compute the dynamic gas cost.
+
+- `SetMinStack`: update the minimum number of items in the stack required to
+execute the operation.
+
+- `SetMaxStack`: update the maximum number of items that will be in the stack
+after executing the operation.
+
+- `SetMemorySize`: the memory size required by the operation.
+
+Within the evmOS framework, developers can modify any of the previous properties.
+
+## Improvement Proposals
+
+Improvement proposals are the approach used by evmOS and Ethereum to modify the
+behavior of opcodes. They are composed of a function, which has access
+to the jump table to apply specific changes to operation behavior, and a name.
+
+In the context of Ethereum, these protocol changes are
+named Ethereum Improvement Proposals (EIPs) and are identified by a unique ID.
+For example, [EIP-1559](https://eips.ethereum.org/EIPS/eip-1559) is
+used to introduce the base fee.
+
+To allow any evmOS partner to define their own specific
+improvements without overlapping with evmOS and Ethereum ones, each
+proposal is identified by a string, that is composed of the chain name and a number. For
+example, default evmOS improvements are associated with the string `evmos_XXXX`.
+This allows each chain to define their improvements without having to worry
+about existing or future ID clashes between different chains.
+Additionally, the ability to start enumeration at 0 is better for chain developers
+and allows having a better overview of the historical progress for each chain.
+
+Below, you will find an example of how the Evmos chain uses this functionality to modify the
+behavior of the `CREATE` and `CREATE2` opcodes. First, the modifier function has
+to be defined:
+
+```go
+// Enable0000 contains the logic to modify the CREATE and CREATE2 opcodes
+// constant gas value.
+func Enable0000(jt *vm.JumpTable) {
+    multiplier := 10
+
+	currentValCreate := jt[vm.CREATE].GetConstantGas()
+	jt[vm.CREATE].SetConstantGas(currentValCreate * multiplier)
+
+	currentValCreate2 := jt[vm.CREATE2].GetConstantGas()
+	jt[vm.CREATE2].SetConstantGas(currentValCreate2 * multiplier)
+}
+```
+
+Then, the function as to be associated with a name via a custom activator:
+
+```go
+evmosActivators = map[string]func(*vm.JumpTable){
+    "evmos_0": eips.Enable0000,
+}
+```
+
+## Activation of Improvement Proposals
+
+Due to continuous changes in the users' interaction with the protocol, and to
+introduce a safety measure along with the freedom to customize the virtual
+machine behavior, custom improvement proposals are not active by default.
+The activation of selected improvement proposals is controlled by the [EVM module's parameters](https://github.com/evmos/evmos/blob/main/proto/ethermint/evm/v1/evm.proto#L17-L18).
+There are two ways of introducing the required parameter changes:
+
+1. **Upgrade**: create a protocol upgrade handler which introduces the proposal name
+in the active list.
+
+2. **Governance**: create governance proposal to add an improvement proposal to the EVM module parameters.
+
+This approach gives developers the ability to react to security issues or market conditions,
+while keeping the chain's participants in the loop.
+
+## Additional Resources
+
+1. [Evmos Custom EIPs](https://github.com/evmos/evmos/blob/main/app/eips/README.md):
+please refer to this document for a detailed description of how opcodes and
+custom improvement proposals have to be used in the evmOS framework.