docs: add qgb module readme (#1052)

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## Overview
Closes #501
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Co-authored-by: Rootul P <rootulp@gmail.com>

x/qgb/README.md

@@ -0,0 +1,129 @@
+# `x/qgb`
+
+## Abstract
+
+This module contains the [Quantum Gravity Bridge](https://blog.celestia.org/celestiums/) (QGB) state machine implementation.
+
+## State machine
+
+The QGB state machine handles the creation of [`AttestationRequestI`](https://github.com/celestiaorg/celestia-app/blob/801a0d412631989ce97748badbd7bb676982db16/x/qgb/types/attestation.go#L15-L23) which can be either a [`DataCommitment`](https://github.com/celestiaorg/celestia-app/blob/801a0d412631989ce97748badbd7bb676982db16/proto/qgb/types.proto#L31-L47) or a [`Valset`](https://github.com/celestiaorg/celestia-app/blob/801a0d412631989ce97748badbd7bb676982db16/proto/qgb/types.proto#L17-L29).
+
+During their creation, the state machine might panic due to an unexpected behavior or event. These panics will be discussed below.
+
+### Conditions of generating a new attestation
+
+#### New valset creation
+
+A new valset is created in the following situations:
+
+```golang
+if (latestValset == nil) || (lastUnbondingHeight == uint64(ctx.BlockHeight())) || significantPowerDiff {
+    ...
+}
+```
+
+- No valset exists in store, so a new valset will be created. This happens mostly after genesis, or after a hard fork.
+- The current block height is the last unbonding height, i.e. when a validator is leaving the validator set. A new valset will need to be created to accommodate that change.
+- A significant power difference happened since the last valset. This could happen if a validator has way more staking power or the opposite. The significant power difference threshold is defined by the constant `SignificantPowerDifferenceThreshold`, and is set to 5% currently.
+
+#### New data commitment creation
+
+A new data commitment is created in the following situation:
+
+```golang
+if ctx.BlockHeight() != 0 && ctx.BlockHeight()%int64(k.GetDataCommitmentWindowParam(ctx)) == 0 {
+	...
+}
+```
+
+I.e. the current block height is not 0, and we're at a data commitment window height.
+
+The data commitment window is defined as a governance parameter that can be changed. Currently, it is set to 400.
+
+### Panics
+
+During EndBlock step, the state machine generates new attestations if needed. During this generation, the state machine could panic.
+
+#### Valset panics
+
+When checking if the state machine needs to generate a new valset, the state machine might panic if it finds invalid state. This can happen in the following cases:
+
+- When checking that a previous valset has been emitted, but it is unable to get it:
+
+```golang
+if k.CheckLatestAttestationNonce(ctx) && k.GetLatestAttestationNonce(ctx) != 0 {  
+   var err error  
+   latestValset, err = k.GetLatestValset(ctx)  
+   if err != nil {  
+      panic(err)  
+   }  
+}
+```
+
+- When getting the current valset:
+
+```golang
+vs, err := k.GetCurrentValset(ctx)
+if err != nil {  
+   // this condition should only occur in the simulator  
+   // ref : https://github.com/Gravity-Bridge/Gravity-Bridge/issues/35 
+   if errors.Is(err, types.ErrNoValidators) {  
+      ctx.Logger().Error("no bonded validators",  
+         "cause", err.Error(),  
+      )  
+      return  
+   }  
+   panic(err)  
+}
+```
+
+- When creating the internal validator struct, i.e. mapping the validators EVM addresses to their powers:
+
+```golang
+intLatestMembers, err := types.BridgeValidators(latestValset.Members).ToInternal()  
+if err != nil {  
+   panic(sdkerrors.Wrap(err, "invalid latest valset members"))  
+}
+```
+
+#### Attestations panics
+
+When storing a new attestation, the state machine can panic if it finds invalid state. This latter can happen in the following cases:
+
+- The attestation request created is a duplicate of an existing attestation:
+
+```golang
+key := []byte(types.GetAttestationKey(nonce))  
+store := ctx.KVStore(k.storeKey)  
+
+if store.Has(key) {  
+   panic("trying to overwrite existing attestation request")  
+}
+```
+
+- An error happened while marshalling the interface:
+
+```golang
+b, err := k.cdc.MarshalInterface(at)  
+if err != nil {  
+   panic(err)  
+}
+```
+
+- The universal nonce was not incremented correctly by 1:
+
+```golang
+if k.CheckLatestAttestationNonce(ctx) && k.GetLatestAttestationNonce(ctx)+1 != nonce {  
+   panic("not incrementing latest attestation nonce correctly")  
+}
+```
+
+## Useful links
+
+The smart contract implementation is in [quantum-gravity-bridge](https://github.com/celestiaorg/quantum-gravity-bridge/).
+
+For QGB v2, the orchestrator and relayer implementations will be in a separate repository. Links will be added subsequently.
+
+QGB v1 implementation, including the orchestrator and relayer, is in the [qgb-integration](https://github.com/celestiaorg/celestia-app/tree/qgb-integration) branch.
+
+QGB ADRs are in the [docs](https://github.com/celestiaorg/celestia-app/tree/main/docs/architecture).