docs: add contributing guidelines

CONTRIBUTING.md

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+# Contributing Guidelines
+
+## Environment Setup
+
+Everything is built and tested using Java 8 so please make sure that you are using that version (e.g. `echo $JAVA_HOME` should print something like `/usr/lib/jvm/java-8-openjdk-amd64`).
+
+## Directory Structure
+
+```
+├── build/              # gitignored - build artifacts are put here
+├── build.gradle        # Gradle build file - https://docs.gradle.org/current/userguide/build_file_basics.html
+├── CONTRIBUTING.md     # This file :)
+├── docs/               # gitignored - generated javadocs are stored here by the CD process
+├── gradle/             # autogenerated - contains the gradle wrapper jar and properties
+├── gradlew             # autogenerated - gradle wrapper file for Unix/macOS
+├── gradlew.bat         # autogenerated - gradle wrapper file for Windows
+├── gradle.properties   # config file that contains the name of the repo and the version number
+├── LICENSE.md          # autogenerated - use ./gradlew markdown to create it
+├── LICENSE.mdt         # template for the LICENCE
+├── README.md           # autogenerated - use ./gradlew markdown to create it
+├── README.mdt          # template for the README
+└── src
+    ├── integration     # end to end tests that run the source code against the production API
+    ├── main            # source code for the SDK itself
+    └── test            # unit tests for various portions of the SDK
+```
+
+## Tests
+
+### Unit
+
+Running the unit tests is as simple as running `./gradlew test`
+
+### Integration
+
+Running the integration tests requires some set up since we're actually hitting the production API.
+
+1. Create a / Sign into your developer account at https://dashboard.smartcar.com/login
+2. Reach out to Smartcar to get webhooks enabled for your account
+3. Add the following redirect URI to your application: `https://example.com/auth`
+4. Set the following environment variables
+
+   - `E2E_SMARTCAR_AMT` (fetch from the "Configuration" page)
+   - `E2E_SMARTCAR_CLIENT_ID` (fetch from the "Configuration" page)
+   - `E2E_SMARTCAR_CLIENT_SECRET` (fetch from the "Configuration" page)
+   - `E2E_SMARTCAR_WEBHOOK_ID` (fetch from "Webhooks" page, webhook does not have to be verified for the tests)
+
+5. Run the tests using `./gradlew integration`
+
+### Both
+
+You can run the entire test suite by running `./gradlew check`
+
+## Local Publishing
+
+If you would like to use the SDK the same way that a consumer would, you can actually publish it to a local Maven repository and then use it in another project for testing. This is great for making sure the experience for SDK consumers works as you want it to.
+
+### Setup
+
+1. Create a PGP key pair that can be used to sign the artifact (you only need to do this once)
+
+   ```bash
+   gpg --batch --generate-key <<EOF
+     Key-Type: RSA
+     Key-Length: 2048
+     Name-Real: Smartcar Testing
+     Name-Comment: use only for testing
+     Name-Email: [email protected]
+     Expire-Date: 0
+     Passphrase: testpass1
+     %commit
+     %echo done
+   EOF
+   ```
+
+2. Set the following environment variables so Gradle knows what key to use for signing your release
+
+   ```bash
+   export ORG_GRADLE_PROJECT_signingKey="$(gpg --armor --export-secret-key)"
+   export ORG_GRADLE_PROJECT_signingPassword='testpass1'
+   ```
+
+3. Run `./gradlew publishToMavenLocal` to publish the changes to your local Maven repository (located at `~/.m2/repository`).
+
+4. You can now use your locally published version of the SDK in a project (e.g. [getting-started-java-sdk](http://github.com/smartcar/getting-started-java-sdk)) to test out the changes or view the actually files that will be published by navigating to the `~/.m2` directory. To use the SDK make the following edits to `build.gradle` where you would like to use the SDK (i.e. don't make these edits in the `build.gradle` file in this repository itself)
+
+   1. Add `mavenLocal` before all other repositories
+
+      ```groovy
+      repositories {
+          mavenLocal()
+          mavenCentral()
+      }
+      ```
+
+   2. Update the version of the package used to the version that you published locally
+      ```groovy
+      dependencies {
+        compile 'com.smartcar.sdk:java-sdk:<the version you have locally>'
+      }
+      ```
+
+## Workflow
+
+1. Set up your development environment
+2. Make changes to main / test / integration as needed to implement your changes
+3. Run unit and integration tests
+4. Iterate on your changes
+5. Publish the SDK locally and verify your changes with some sample code
+6. Iterate on your changes further if needed
+7. Generate and review the JavaDoc (e.g. `./gradlew javadoc && python -m http.server --directory docs`) (note: This is for local verification only and does not need to be committed, the CD process will generate the ones that are published).
+8. Bump the version number in `gradle.properties` and regenerate the README by running `./gradlew markdown`)
+9. Commit the version number and README changes and open a PR :tada: