[FLINK-23448] update README

This closes ververica#30

CONTRIBUTING.md

@@ -0,0 +1,107 @@
+# How to contribute to this project
+
+> **:heavy_exclamation_mark: Important:** This section contains tips for developers who are
+> maintaining the `flink-training` project (not so much people doing the training).
+
+The following sections apply on top of the [Setup Instructions](README.md#set-up-your-development-environment) above.
+
+## Format code
+
+Just like [Apache Flink](https://github.com/apache/flink), we use the [Spotless
+plugin](https://github.com/diffplug/spotless/tree/main/plugin-maven) together
+with [google-java-format](https://github.com/google/google-java-format) to
+format our Java code. You can configure your IDE to automatically apply
+formatting upon saving with these steps:
+
+1. Install the [google-java-format
+   plugin](https://plugins.jetbrains.com/plugin/8527-google-java-format) and
+   enable it for the project
+2. In the plugin settings, change the code style to "AOSP" (4-space indents)
+3. Install the [Save Actions
+   plugin](https://plugins.jetbrains.com/plugin/7642-save-actions)
+4. Enable the plugin, along with "Optimize imports" and "Reformat file" but
+   ignoring `.*README\.md`.
+
+## Ignore refactoring commits
+
+There is a list of refactoring commits in `.git-blame-ignore-revs`.
+When looking at change annotations using `git blame`, it is helpful to ignore these.
+You can configure git and your IDE to do so with:
+
+```bash
+git config blame.ignoreRevsFile .git-blame-ignore-revs
+```
+
+## :heavy_plus_sign: Add new exercises :heavy_plus_sign:
+
+To add a new exercise, we recommend copying an existing one and adapting it. Make sure the new subproject's `build.gradle` file
+contains appropriate class name properties so that we can create the right tasks for
+[running tests and solutions on the command line](README.md#running-exercises-tests-and-solutions-on-the-command-line).
+
+For example:
+
+```groovy
+ext.javaExerciseClassName = 'org.apache.flink.training.exercises.ridesandfares.RidesAndFaresExercise'
+ext.scalaExerciseClassName = 'org.apache.flink.training.exercises.ridesandfares.scala.RidesAndFaresExercise'
+ext.javaSolutionClassName = 'org.apache.flink.training.solutions.ridesandfares.RidesAndFaresSolution'
+ext.scalaSolutionClassName = 'org.apache.flink.training.solutions.ridesandfares.scala.RidesAndFaresSolution'
+
+apply plugin: 'application'
+
+mainClassName = ext.javaExerciseClassName
+```
+
+## Useful Gradle Commands and Tricks
+
+### Clean build with all checks
+
+```bash
+./gradlew clean check shadowJar
+./gradlew clean check shadowJar --no-build-cache
+```
+
+### Force a re-run of all tests only
+
+```bash
+./gradlew cleanTest test  --no-build-cache
+```
+
+> **:information_source: Note:** Ignoring the build-cache is required if you really want to run the test again
+> (without any changes in code). Otherwise the test tasks will just pull the latest test results from the cache.
+
+### Fix formatting
+
+```bash
+./gradlew spotlessApply
+./gradlew :rides-and-fares:spotlessApply
+```
+
+> **:information_source: Note:** You do not have to remember this since `./gradlew check` will not only
+> verify the formatting and print any errors but also tell you the command to use to fix these.
+
+### Tune the Java compiler
+
+Add the following code to the `subprojects { /*...*/ }` section of the
+[`build.gradle`](build.gradle) file and adapt accordingly. For example:
+
+```groovy
+    tasks.withType(JavaCompile) {
+        options.compilerArgs << '-Xlint:unchecked'
+        options.deprecation = true
+    }
+```
+
+> **:information_source: Note:** We do not add this by default to keep the training
+> requirements low for participants and focus on the exercises.
+
+### :warning: Deprecated Gradle Features
+
+You may see this warning being reported from Gradle:
+> Deprecated Gradle features were used in this build, making it incompatible with Gradle 8.0.
+>
+> You can use '--warning-mode all' to show the individual deprecation warnings and determine if they come from your own scripts or plugins.
+>
+> See https://docs.gradle.org/7.1/userguide/command_line_interface.html#sec:command_line_warnings
+
+This is currently caused by https://github.com/johnrengelman/shadow/issues/680
+and has to be fixed by the shadow plugin developers.