Add contributing instructions. (#429)

* Add contributing instructions.

* Resolve comments.

CONTRIBUTING.md

@@ -0,0 +1,64 @@
+# Contributing to Java Debugger
+
+## Build and Debug
+
+### Getting the source
+This debugger is written in [TypeScript](https://github.com/Microsoft/TypeScript), and it depends on a [Java Debug Server](https://github.com/Microsoft/java-debug) written in Java.
+- Suggest to create a new folder first.
+  ```bash
+  mkdir javaDebugger
+  cd javaDebugger
+  ```
+- Check out source code for the extension.
+  ```bash
+  git clone https://github.com/Microsoft/vscode-java-debug.git
+  ```
+- Check out source code for the debug server.
+  ```bash
+  git clone https://github.com/Microsoft/java-debug.git
+  ```
+Now the folder structure looks like following:
+```bash
+javaDebugger/
+├── java-debug
+└── vscode-java-debug
+```
+
+### Prerequisites
+- [JDK](http://www.oracle.com/technetwork/java/javase/downloads/index.html), (version 1.8.0 or later)
+- [VS Code](https://code.visualstudio.com/), (version 1.19.0 or later)
+- [Node.JS](https://nodejs.org/en/), (>= 8.9.1, < 9.0.0)
+- [Language Support for Java by Red Hat](https://marketplace.visualstudio.com/items?itemName=redhat.java), (version 0.14.0 or later)
+
+Install all the dependencies using `npm` (supposed to be installed together with [Node.JS](https://nodejs.org/en/)).
+```bash
+cd vscode-java-debug
+npm install
+```
+
+### Build and Run
+#### Build the Debug Server
+For convenience, there is task `build_server` defined in `gulpfile.js`. It builds the Java Debug Server and then copies the .jar file into folder `vscode-java-debug/server`.
+```bash
+npx gulp build_server
+```
+**NOTE**: If you didn't follow the steps to check out [vscode-java-debug](https://github.com/Microsoft/vscode-java-debug) and [java-debug](https://github.com/Microsoft/java-debug) in the same folder, please specify a correct `server_dir` in your [gulpfile.js](https://github.com/Microsoft/vscode-java-debug/blob/master/gulpfile.js#L5).
+
+#### Debug the Extension
+Open folder `vscode-java-debug` in VS Code, or simply execute following commands if you have `code` in your system PATH.
+```bash
+cd vscode-java-debug
+code .
+```
+Press <kbd>F5</kbd> to start debugging the extension, it will create a new window as the extension host.
+
+#### Debug the Debug Server
+When you are debugging the extension, it is able to debug the Java process with local port `1044`. To remote debug the server, you can attach a Java debugger to `localhost:1044` using an IDE (Eclipse, IntelliJ IDEA, etc) or the Java Debugger for VS Code itself.
+
+Since we have checked in a valid [launch.json](https://github.com/Microsoft/java-debug/blob/master/.vscode/launch.json) to the repository, it would be easy to use the Java Debugger for VS Code itself to debug the server.
+- Open folder `java-debug` in a new window in VS Code.
+- Press <kbd>F5</kbd> to attach.
+
+## Pull Requests
+Before we can accept a pull request from you, you'll need to sign a [Contributor License Agreement (CLA)](https://github.com/Microsoft/vscode/wiki/Contributor-License-Agreement). It is an automated process and you only need to do it once.
+To enable us to quickly review and accept your pull requests, always create one pull request per issue and [link the issue in the pull request](https://github.com/blog/957-introducing-issue-mentions).