Add document about how kustomize build works

docs/howItWorks.md

@@ -0,0 +1,51 @@
+# How does `kustomize build` work
+
+Call stack when running `kustomize build`, with links to code.
+
+## Run build
+
+* [RunBuild](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/kustomize/internal/commands/build/build.go#L121)
+  * [MakeKustomizer](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/krusty/kustomizer.go#L32)
+  * [Run](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/krusty/kustomizer.go#L47): performs a kustomization. It uses its internal filesystem reference to read the file at the given path argument, interpret it as a kustomization.yaml file, perform the kustomization it represents, and return the resulting resources.
+    * Create factories
+      * [tranformer.NewFactoryImpl](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/k8sdeps/transformer/factory.go#L17)
+      * [resmap.NewFactory](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/resmap/factory.go#L21)
+        * [resource.NewFactory](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/resource/factory.go#L23)
+          * [kustruct.NewKunstructuredFactoryImpl](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/k8sdeps/kunstruct/factory.go#L28)
+      * [loader.NewLoader](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/loader/loader.go#L19)
+      * [validator.NewKustValidator](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/k8sdeps/validator/validators.go#L23)
+    * [NewKustTarget](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L38)
+    * [Load](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L54)
+    * [MakeCustomizeResMap](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L109): details in next section
+  * [emitResources](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/kustomize/internal/commands/build/build.go#L143)
+
+## Make resource map
+
+* [makeCustomizeResMap](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L117)
+  * [AccumulateTarget](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L196): returns a new ResAccumulator, holding customized resources and the data/rules used to do so. The name back references and vars are not yet fixed.
+    * [accummulateResources](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L302): fills the given resourceAccumulator with resources read from the given list of paths.
+    * Merge config from builtin and CRDs
+    * [runGenerators](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L239)
+      * [configureBuiltinGenerators](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget_configplugin.go#L28)
+        * ConfigMapGenerator
+        * SecretGenerator
+      * [configureExternalGenerators]()
+      * Iterate all generators
+    * [runTransfomers](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L274)
+      * [configureBuiltinTransformers](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget_configplugin.go#L44)
+        *	PatchStrategicMergeTransformer
+        * PatchTransformer
+        * NamespaceTransformer
+        * PrefixSuffixTransformer
+        * LabelTransformer
+        * AnnotationsTransformer
+        * PatchJson6902Transformer
+        * ReplicaCountTransformer
+        * ImageTagTransformer
+      * [configureExternalTransformers](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L291)
+    * [MergeVars](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/accumulator/resaccumulator.go#L64)
+  * The following steps must be done last, not as part of the recursion implicit in AccumulateTarget.
+    * [addHashesToNames](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L153)
+    * [FixBackReferences](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/accumulator/resaccumulator.go#L160): Given that names have changed (prefixs/suffixes added), fix all the back references to those names.
+    * [ResolveVars](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/accumulator/resaccumulator.go#L141)
+    * [computeInventory](https://github.com/kubernetes-sigs/kustomize/blob/c7d78970fb86782dbdded3a93944b774f826071f/api/internal/target/kusttarget.go#L163)