TASK: Add docs for contribution getting started (#3798)

* TASK: Add docs for contribution getting started

* TASK: Adds code-style

---------

Co-authored-by: Markus Günther <[email protected]>

README.md

@@ -48,7 +48,7 @@ And on Packagist available via: `neos/neos-ui`
 
 ### Updating
 
-```
+```bash
 composer update neos/neos-ui
 ```
 
@@ -58,14 +58,32 @@ For trying out the new UI, we recommend you to run the regularily released beta
 However, if you want to stay on bleeding-edge, or want to help out developing, you'll
 need the `8.3.x-dev` release. You can install the latest release using:
 
-```
+```bash
 composer require neos/neos-ui-compiled:8.3.x-dev neos/neos-ui:8.3.x-dev
 ```
 
 ## Contributing
 
 Please follow the respective guides for contributing on OSX and on Linux.
 
+To start developing the Neos Ui you will need a running Neos instance locally.
+You can use 
+* one of your own, local Neos 8.3 instances,
+* create a new one with `composer create-project neos/neos-base-distribution neos-ui-development-instance`,
+* or use the docker compose setup in this repository (see instructions below).
+
+### Setup Source Files and Git
+To install the source files and setup git, run:
+
+```bash
+composer require neos/neos-ui-compiled:8.3.x-dev neos/neos-ui:8.3.x-dev --prefer-source
+```
+
+This will sync the git repository of Neos Ui into `Packages/Application/Neos.Neos.Ui` (this might take a while).
+To push your changes to GitHub you need to fork the Neos Ui and change the git remote to your fork (check with `git remove -v`).
+
+Run `make setup`. To check what commands are executed have a look at the `Makefile` in the root of this repository.
+
 ### on Windows
 
 1) Ensure you have the relevant version installed (see above).