Local Source Upload Documentation.

Co-authored-by: Rolfe Dlugy-Hegwer <rolfedh@users.noreply.github.com>

docs/local_source_upload.md

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+Local Source Upload
+-------------------
+
+The purpose of project Shipwright is to transform source code into container images, using known strategies to accomplish this task.
+
+Now, with local source upload, we move Shipwright closer to the developer's inner loop. As a developer, you can use the local source upload feature to stream local content to a Build Controller running in a Kubernetes cluster and create a container image from it. This way, you can try out Shipwright before submitting a pull-request and use the cluster's computing power to build the image.
+
+# Usage
+
+To build an image by using the local source upload feature:
+
+1. Create a Build or use a pre-existing one. Register the standard settings, such as the Build Strategy, on the Build resource. For example:
+
+```bash
+shp build create sample-nodejs \
+    --source-url="https://github.com/shipwright-io/sample-nodejs.git" \
+    --output-image="docker.io/<namespace>/sample-nodejs:latest"
+```
+
+2. Clone the repository or use a pre-existing one. For example:
+
+```bash
+git clone https://github.com/shipwright-io/sample-nodejs.git && \
+    cd sample-nodejs
+```
+
+3. Keep working on the project. When you're ready to build or rebuild a container image with the local changes, run:
+
+```bash
+shp build upload sample-nodejs --follow \
+    --output-image="docker.io/<namespace>/sample-nodejs:<tag>"
+```
+
+Notes: 
+- In the preceding examples, replace placeholders like `<namespace>` and `<tag>` with proper values.
+- For `--output-image`, you can specify any Container Registry, use it in combination with `--output-credentials-secret` when needed.
+
+# Streaming
+
+The subcommand `build upload` creates a new `BuildRun` for the informed `Build`. The newly created `BuildRun` contains settings to instruct the Build Controller to wait for the local user upload instead of cloning the external repository as usual.
+
+The command-line interface orchestrates the process of making the `BuildRun`'s Pod wait, and streaming the specified directory when the Pod is ready for it.
+
+The data streamed to the cluster skips the `.git` directory, if present, and any entries specified by the `.gitignore` file.