docs: add instructions to change DefaultGenesis (backport #21680) (#2…

…1991)

Co-authored-by: Eric Mokaya <[email protected]>

docs/build/building-apps/06-app-go-genesis.md

@@ -0,0 +1,47 @@
+---
+sidebar_position: 1
+---
+
+### Modifying the `DefaultGenesis`
+
+It is possible to modify the DefaultGenesis parameters for modules by wrapping the module, providing it to the `*module.Manager` and injecting it with `depinject`.
+
+Example ( staking ) :
+
+```go
+type CustomStakingModule struct {
+    staking.AppModule
+    cdc codec.Codec
+}
+
+// DefaultGenesis will override the Staking module DefaultGenesis AppModuleBasic method.
+func (cm CustomStakingModule) DefaultGenesis() json.RawMessage {
+    params := stakingtypes.DefaultParams()
+    params.BondDenom = "mydenom"
+
+    return cm.cdc.MustMarshalJSON(&stakingtypes.GenesisState{
+        Params: params,
+    })
+}
+
+// option 1 ( for depinject users ): override previous module manager
+depinject.Inject(
+// ... provider/invoker/supplier
+&moduleManager,
+)
+
+oldStakingModule,_ := moduleManager.Modules()[stakingtypes.ModuleName].(staking.AppModule)
+moduleManager.Modules()[stakingtypes.ModuleName] = CustomStakingModule{
+	AppModule: oldStakingModule,
+	cdc: appCodec,
+}
+
+// option 2 ( for non depinject users ): use new module manager
+moduleManager := module.NewManagerFromMap(map[string]appmodule.AppModule{
+stakingtypes.ModuleName: CustomStakingModule{cdc: appCodec, AppModule: staking.NewAppModule(...)},
+// other modules ...
+})
+
+// set the module manager
+app.ModuleManager = moduleManager
+```