Improved documentation for contribution process (#940)

CONTRIBUTING.md

@@ -11,10 +11,23 @@ When using the Save Actions Plugin, you have to copy the *saveactions_settings.x
 
 Furthermore, please import the `/codequality/idea/code_style.xml` file into the project's settings of IntelliJ in order to match the code style we're using.
 
-## Commit Message Policy
+## Process
 
-If you are committing a change which refers to a certain issue, please reference the issue by using the following notation:
+The first step in order to contribute is to fork this repository into your account.
+You can develop your changes on a separate branch there and open a [pull request](https://github.com/inspectIT/inspectit-ocelot/pulls)
+in the main repository as soon as you are done.
 
-`Closes #{Issue ID} - {Issue Description} (#{Pull Request ID})`
+Usually, a pull request should correspond to an open [issue](https://github.com/inspectIT/inspectit-ocelot/issues) in this repository.
+Therefore make sure that an issue exists which your pull request attempts to resolve. If none exists, you should create one.
+There is an exemption to this rule: For very small fixes (e.g. Typo-Fixes), your pull request does not need to have a corresponding issue.
+If your PR corresponds to an issue, the issue should be correctly labeled (e.g. `enhancement`, `bug`, etc.) because the release notes will be generated based on these labels.
 
-A example would be: `Closes #1 - Add README (#2)`
+When you open your pull request, you should use the following format for the title:
+
+`Closes #{Issue ID} - {Description} (#{Pull Request ID})`
+
+Members from the inspectIT team will then review your changes and either request changes or approve your PR.
+In case changed were requested, please fix them (or discuss the changes) and then your PR will be reviewed again.
+
+As soon as everything is approved, the inspectIT admins will merge your pull request.
+Your change will become a single commit with the title of the PR as commit message.