Add development readme

DEVELOPMENT.md

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+# Development
+
+## Getting Started
+
+Before you get started, we recommend installing [Node Version Manager](https://github.com/nvm-sh/nvm#installing-and-updating) to help manage `node` and `npm` versions. Next, from your local copy of the action run `nvm use` and `npm install`. You're ready to start coding!
+
+## Git Workflows
+
+We use the [feature branch workflow](https://www.atlassian.com/git/tutorials/comparing-workflows/feature-branch-workflow). The workflow for a typical code change looks like this:
+
+1. Create a new branch for the feature.
+2. Make changes to the code.
+3. Use `npx changeset` to create a changeset describing the change to users.
+4. Commit your changes.
+5. Open a pull request to the `main` branch.
+6. Once all checks are passing and the PR is approved, Squash and Merge into the `main` branch.
+
+## Creating a release
+
+We use [Changesets](https://github.com/changesets/changesets) to automate versioning and releasing. When you are ready to release, the first step is to create the new version.
+
+1. Go to pull requests and view the "Version Action" PR.
+2. Review the PR:
+    - [ ] Changelog entries were created.
+    - [ ] Version number in package.json was bumped.
+    - [ ] All `.changeset/*.md` files were removed.
+3. Approve, then "Squash and merge" the PR into `main`.
+
+Merging the versioning PR will run a workflow that creates or updates all necessary tags. It will also create a new release in GitHub.
+
+## Managing the Dockerfile & Docker Image
+
+The base of the `Dockerfile` is hosted as a Docker image on DockerHub. The image is originally built locally from `Dockerfiles/Dockerfile` and pushed up to DockerHub.
+
+The main `Dockerfile` at the project root imports the `wpengine/site-deploy:base-stable` hosted image from DockerHub.
+
+All other customizations that are updated less frequently, or managed by 3rd parties, should be added to the `base-stable` image. Rebuild and push the image to DockerHub to update the image.
+
+## Updating the Docker Image
+The `base-stable` Docker Image will rarely need to be updated, however it may be necessary to update it manually* from time to time.
+
+- Build the docker image locally:
+`docker build --no-cache -t wpengine/site-deploy:base-stable Dockerfiles`
+
+- Push the image to DockerHub:
+`docker push wpengine/site-deploy:base-stable`
+
+Once the hosted Docker image is updated, it will need to be imported (or updated) on the main project `Dockerfile`. If the Dockerhub tag name (image version) has changed, update the existing line in the project root `Dockerfile` to match the new tag name.
+- Update the root Docker file for the project with the latest version of the Docker Image:tagname. i.e.:
+`FROM wpengine/site-deploy:base-stable`