[ggj][docs] Add development workflow docs

DEVELOPMENT.md

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+# Development Workflow
+
+This is a temporal workflow that will evolve with codebase. The steps described
+below are temporary and better ones will be coming.
+
+## Running the Plugin
+
+1.  Clone [googleapis](https://github.com/googleapis/googleapis) and
+    [gapic-showcase](https://github.com/googleapis/gapic-showcase/) and install
+    protoc.
+2.  Copy the protos from Showcase into googleapis/google/showcase.
+
+    ```sh
+    cp gapic-showcase/schema/google/showcase/v1beta1 googleapis/google/showcase/v1beta
+    ```
+
+3.  Export tool paths.
+
+    ```sh
+    PROTOC_INCLUDE_DIR=/usr/local/include/google/protobuf/
+    GOOGLEAPIS_DIR=/usr/local/google/home/$USER/dev/googleapis
+    YOUR_PROTO_DIR=/usr/local/google/home/USER/dev/googleapis/google/showcase/v1beta
+    ```
+
+4.  Build this plugin.
+
+    ```sh
+    bazel build :protoc-gen-gapic-java
+    ```
+
+5.  Run the plugin. At this stage, it will not do anything except write
+    hardcoded Java into two files.
+
+    ```
+    protoc -I=${PROTOC_INCLUDE_DIR} -I=${GOOGLEAPIS_DIR} -I=${YOUR_PROTO_DIR} \
+        --plugin=bazel-bin/protoc-gen-gapic-java ~/dev/googleapis/google/showcase/v1test/*.proto \
+        --gapic-java_out=/tmp/test
+    ```
+
+## Code Formatting
+
+-   Run linter checks without actually doing the formatting.
+
+    ```sh
+    bazel build :google_java_format_verification
+    ```
+
+-   Format files.
+
+    ```sh
+    bazel run :google_java_format
+    ```