chore: add fees documentation (#120)

Co-authored-by: Kirill <[email protected]>

Author: MakMuftic

README.md

@@ -16,6 +16,7 @@ Sygma bridge uses [chainbridge-core](https://github.com/ChainSafe/chainbridge-co
 
 1. [Local environment](#local-environment)
 2. [Configuration](#configuration)
+3. [Technical documentation](#technical-documentation)
 
  
 
@@ -50,3 +51,8 @@ translate to ENV variable named `SYG_RELAYER_MPCCONFIG_PORT`.
 
 Each `ChainConfig` is defined as one ENV variable, where its content is JSON configuration for one chain/domain.
 Variables are named like this: `SYG_DOM_X` where `X` is domain id.
+
+## Technical documentation
+Each service has a technical documentation inside its repository under `/docs` directory. [Here](/docs/Home.md) you can find technical documentation for relayers.
+
+Additionally, the [general high-level documentation for the entire Sygma system](/docs/general/Arhitecture.md) can be found in this same repository under the `/docs/general` directory.

docs/Home.md

@@ -0,0 +1,13 @@
+# Relayers
+
+_Relayers are entities that ensure bridge decentralization and execute asset bridging._
+
+## Abstract
+
+At the core of Sygma exists a set of relayers that are distributed among several legal entities and operate under a
+trusted federation model. Spreading the relayer set across several legal entities creates a distribution of
+responsibilities and mitigates risk by disincentivizing relayers in the set to act in an unfair manner.
+Each relayer within the set listens to both the source and destination chains that are being bridged by Sygma. Based on
+events that are emitted, these relayers then execute relevant actions.
+This multi-relayer set is responsible for relaying messages from a source network to a destination network. Relayers are
+operating with private key share and execution happens on the destination network with MPC private key.

docs/general/Arhitecture.md

@@ -0,0 +1,8 @@
+# Sygma Architecture
+
+![](/docs/resources/sygma-arhitecture.png)
+
+## Components
+
+- **[Fees](/docs/general/Fees.md)** - high-level overview of handling fees
+- **[Relayers](/docs/Home.md)** - relayer technical documentation

docs/general/Fees.md

@@ -0,0 +1,69 @@
+# Fees
+
+Sygma allows granular control of handling fees for each resource. Even though specific implementations differ based on
+chain architecture, general functionality is the same. The concept is that each resource is assigned a fee strategy for
+every potential destination domain, with this mapping also outlining all potential bridging routes for a given resource.
+
+![](/docs/resources/fee-router-general.png)
+
+## Fee strategies
+
+Fee strategy defines a set of rules on how fees should be charged when executing deposits on the source chain.
+
+### Static fee strategy
+
+This strategy always requires a predefined static fee amount per deposit. **It can only collect fees in the native
+currency of the source chain**.
+
+*On the diagram below, we use [Sygma SDK](https://github.com/sygmaprotocol/sygma-sdk) for interaction with all services.*
+
+![](/docs/resources/static-fee-general.png)
+
+#### Deposit flow
+
+1) Calculate the final fee
+    - Based on resourceID and domainsID, request a final fee amount that will be required to execute the deposit.
+2) Execute deposit
+    - Send the appropriate base currency amount based on the calculated final fee when executing the deposit.
+
+### Dynamic fee strategy
+
+This strategy utilizes the [Sygma Fee Oracle service](https://github.com/sygmaprotocol/sygma-fee-oracle), which issues
+fee estimates with details on the gas price for the destination chain. In addition, fee oracle can provide price
+information for different tokens, enabling paying bridging fees in the not native currency. Each issued gas estimate has
+a limited time validity in which it needs to be executed.
+
+Check out
+the [Sygma Fee Oracle technical documentation](https://github.com/sygmaprotocol/sygma-fee-oracle/blob/main/docs/Home.md) for
+more details on the service and the format of the issued fee estimates.
+
+*On the diagram below, we use [Sygma SDK](https://github.com/sygmaprotocol/sygma-sdk) for interaction with all services.*
+
+![](/docs/resources/dynamic-fee-general.png)
+
+#### Deposit flow
+
+1) Fetch fee estimate
+    - Based on resource ID and domains ID, request fee estimate from Fee Oracle service. This fee estimate is valid
+      until `expiresAt`.
+2) Validate fee estimate and calculate the final fee
+    - Validate the signature on the fee estimate.
+    - Get the final fee amount that will be collected on deposit.
+3) Execute deposit
+    - Provide fee estimate data as an argument when executing the deposit.
+
+## Implementations
+
+#### EVM
+
+Check out the [solidity documentation](https://github.com/sygmaprotocol/sygma-solidity/blob/main/docs/Home.md) for
+details on EVM implementation.
+
+#### Substrate
+
+Check out
+the [substrate pallet documentation](https://github.com/sygmaprotocol/sygma-substrate-pallets/blob/main/docs/Home.md)
+for details on Substrate implementation.
+
+
+