[meta] Create contributor intro (closes javalin#233)

CONTRIBUTING.md

@@ -12,12 +12,80 @@ Every PR will be considered.
 * Ask about the feature beforehand (or pick one of the open issues)
 * If no issue exists, create an issue for the PR
 * Try to write some tests for your change. There are a lot of examples in the `test` dir.
-* Format your code so it looks somewhat like the rest of the source (which is formatted using default settings in IntelliJ)
+* Format your code so it looks somewhat like the rest of the source 
+  (which is formatted using default settings in IntelliJ)
 
-## Runnning tests
+## Running tests
 
 `mvn test`
 
 ## Building project locally
 
 `mvn install`
+
+## Project overview
+
+Javalin is a fairly small project (less than 4k LoC, including comments).
+
+### Package overview
+
+The following ASCII file structure diagram shows the most important packages and files, with comments.
+
+```
+io.javalin
+├── apibuilder/               // Convenience methods for when you're creating applications with a lot of routes
+├── core/
+│   ├── util/
+│   │   ├── JettyServerUtil   // Responsible for setting up the Jetty Server
+│   ├── JavalinServlet.kt     // Responsible for the request lifecycle (writes the response to the client)
+│   └── PathMatcher.kt        // Responsible for matching requests and parsing URLs
+├── json/                     // JSON mapping (functional interfaces + optional Jackson implementation)
+├── rendering/                // Template engines and markdown renderer
+├── security/                 // All functionality related to access management for routes
+├── staticfiles/              // All functionality related to static files
+├── websocket/
+│   ├── WsSession.kt          // Similar to Context.kt, but for WebSockets
+│   └── WsPathMatcher.kt      // Similar to PathMatcher.kt, but for WebSockets
+├── Context.kt                // Wrapper class for request/reponse, contains everything needed to fulfill a request
+└── Javalin.java              // Main public API, responsible for setting up and configuring the server
+```
+
+Most of the codebase is Kotlin (~2.5k lines), and the rest is Java (~1k lines). 
+Any public API which takes a `@FunctionalInterface` as a parameter has to be Java 
+to achieve the best interoperability between Java and Kotlin. 
+Every `@FunctionalInterface` is also Java for the same reasons. 
+All tests are written in Kotlin.
+
+### Test setup
+
+Almost all tests should be full integration tests. This means every test should
+
+* Start a Javalin instance on a random port
+* Attach a handler which does what you want to test
+* Perform a HTTP request against that handler
+* Verify what you want to test using the response value
+* Stop the Javalin instance
+
+This is a lot easier than it sounds. Here is an example test:
+
+```kotlin
+@Test
+fun `session-cookie is http-only`() = TestUtil.test { app, http ->
+    app.get("/store-session") { ctx -> ctx.sessionAttribute("test", "tast") }
+    assertThat(http.get("/store-session").headers.getFirst("Set-Cookie").contains("HttpOnly"), `is`(true))
+}
+```
+
+The `TestUtil.test` function takes care of creating, starting and stopping a Javalin instance (on a random port), 
+and provides you with a http-client you can use to perform requests.
+You can also give `test` a Javalin instance:
+
+```
+TestUtil.test(Javalin.create().dontIgnoreTrailingSlashes()) { app, http ->
+```
+
+`test` will assign the instance a random port and stop it after the test is done.
+
+## Questions
+
+There's usually someone on [gitter](https://gitter.im/javalin-io/general) who can help you if you have any questions.