doc: GoDep Design doc (#70)

aws_lambda_builders/workflows/go_dep/DESIGN.md

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+# Go Dep - Lambda Builder
+
+## Scope
+Building Go projects using the dep tool (https://github.com/golang/dep) is rather simple, if you was to do
+this by hand, you would perform these commands:
+
+    - `dep ensure`
+    - `GOOS=linux GOARCH=amd64 go build -o handler main.go`
+    - `zip -r source.zip`
+
+The scope of the Go dep builder is to create a macro for these commands to ensure that spelling and paths are correct.
+We don't have to care about versioning of the tooling of either Go or dep since Lambda doesn't have to care, and so it becomes
+user preference.
+
+## Implementation
+The go-dep builder runs the above commands with some minor tweaks, the commands ran on behalf of the user are:
+
+    1. dep ensure
+    2. GOOS=linux GOARCH=amd64 go build -o $ARTIFACT_DIR/$HANDLER_NAME $SOURCE_DIR
+
+The main difference being we want to capture the compiled binary to package later, so the binary has the
+output path as the artifact dir set by the caller.
+
+## Challenges
+There are no challenges for go building, most problems have been abstracted away by the Go tooling
+
+## Notes
+Go does native cross-compilation regardless of what's compiling it. Regardless of how the user builds their code it would run on
+AWS Lambda.
+
+### Layers
+This pattern might not work for Layers, plugins for go require an extra compilation flag (`-buildmode=plugin`), this would be something
+to add later on, should SAM CLI support building layers