docs: add instructions on how to use the dynamic fee pallet (parityte…

…ch#371)

docs/frame/dynamic-fee.md

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+# Dynamic fee pallet
+
+The dynamic fee pallet allows a Substrate blockchain with Frontier to simulate
+the functionality of Ethereum dynamic fee adjustment.
+
+## Overview
+
+The pallet works by keeping track of the current minimum gas price in a
+Substrate storage `MinGasPrice`. Each Substrate block, the proposer can submit
+an inherent extrinsic `note_min_gas_price_target`. When a block is built, the
+minimum gas price is adjusted similar to Ethereum's algorithm.
+
+## Usage
+
+To use the dynamic fee pallet, first include the pallet in runtime by
+implementing `pallet_dynamic_fee::Config`. The `MinGasPriceBoundDivisor` is a
+divisor used to set how much the minimum gas price is adjusted each block. You
+can set it to `1024` to get the same algorithm as Ethereum. After implementing
+`pallet_dynamic_fee::Config`, include the pallet in the runtime definition.
+
+With the pallet in place, you can now extend the node to allow it to vote on the
+minimum gas price target, via the inherent data providers. Locate in the node
+service code where the `InherentDataProviders` struct is built, and add
+`pallet_dynamic_fee`'s inherent data provider. The inherent data provider
+requires a target gas price parameter to be provided. For that, you simply need
+to add a new custom command line argument.
+
+An example code snippet is shown below:
+
+```rust
+fn inherent_data_providers(
+    target_min_gas_price: U256
+) -> Result<sp_inherents::InherentDataProviders, ServiceError> {
+    let inherent_data_providers = sp_inherents::InherentDataProviders::new();
+
+    if !inherent_data_providers.has_provider(&pallet_dynamic_fee::INHERENT_IDENTIFIER) {
+        inherent_data_providers
+            .register_provider(pallet_dynamic_fee::InherentDataProvider(target_min_gas_price))
+            .map_err(Into::into)
+            .map_err(sp_consensus::Error::InherentData)?;
+    }
+
+    Ok(inherent_data_providers)
+}
+```