add build to project structure

docs/concepts.md

@@ -19,14 +19,17 @@ This is example Mantil project structure with two API's: ping and second.
 │   │   └── ping_test.go
 │   └── second
 │       └── second.go
+├── build
+│   └── functions
+│       ├── ping
+|       |   ├── main.go
+|       |   └── bootstrap
+│       └── second
+|           ├── main.go
+|           └── bootstrap
 ├── config
 │   ├── environment.yml
 │   └── state.yml
-├── functions
-│   ├── ping
-│   │   └── main.go
-│   └── second
-│       └── main.go
 ├── go.mod
 ├── go.sum
 ├── public
@@ -71,9 +74,9 @@ the _/public_ folder and it will be visible at the endpoint root.
 
 ### Build folder
 
-Build folder contains main package for each API which is generated code that transforms your API's to Lambda
-functions. Binaries of your functions are also placed in this folder. It is generated by Mantil and you should not modify it.
-If you're using source control this folder should be untracked by adding it to your _.gitignore_ file since all data in it is generated on each deploy. This is automatically done for you when initializing project with _new_ command.
+Build folder is automatically generated on each deploy and contains main package for each API which is code that transforms your API's to Lambda
+functions. Binaries of your functions named _bootstrap_ are also placed in this folder.
+If you're using source control this folder should be untracked by adding it to your _.gitignore_ file since all data is generated on each deploy. This is automatically done for you when initializing project with _new_ command.
 
 Code in build folder uses [Mantil Go
 SDK](https://github.com/mantil-io/mantil.go) to transform you API's to AWS