Add quick-start-contribution-onboarding.rst and improve gitlab.rst

@kristis and I prepared documentation file which describes how to contribute to the Oniro Project

Closes #150 (closed)

Signed-off-by: Grzegorz Gwóźdź grzegorz.gwozdz@huawei.com

contributing/how-to-contribute.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+.. include:: ./definitions.rst
+
+How to contribute to the |main_project_name|
+############################################
+This page explains how to contribute to the |main_project_name| for
+new people who would like to join.
+
+
+.. contents::
+   :depth: 2
+
+Setting up
+**********
+
+Creating an account on Eclipse 
+------------------------------
+
+Head to the `Eclipse foundation website <https://accounts.eclipse.org/user/register?destination=user/login>`_ and set up an account by entering your:
+
+- Email
+- Username
+- Full name
+- Organization
+- Password
+- Country
+
+Then read and check the box to agree to Terms of Use, Privacy Policy and Code of Conduct. 
+When you complete that, follow the instructions sent to your email to activate the account.
+
+Setting up Gitlab
+-----------------
+
+Now you can go to `the Oniro Gitlab <https://gitlab.eclipse.org/eclipse/oniro-core/oniro>`_ 
+. You should use the account that was created in the previous step to log in.
+
+To push and pull over HTTPS with Git using your account, you must set a password
+or `a Personal Access Token <https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html>`_
+to use instead.
+
+If you want to push or pull repositories using SSH, you have to
+`add a SSH key <https://docs.gitlab.com/ee/user/ssh.html>`_ to your profile.
+
+Signing the ECA
+---------------
+
+In order to contribute to the |main_project_name| you need to sign the `ECA <https://accounts.eclipse.org/user/eca>`_.
+
+Contributing to the |main_project_name|
+***************************************
+
+If you complete previous steps, your account is ready and you can contribute to the |main_project_name|.
+
+Setting up git
+--------------
+
+Make sure to have git installed in your device. After that configure it by
+typing:
+
+.. code-block:: bash
+
+  $ git config user.email "your_eclipse_account_email"
+  $ git config user.name "Your eclipse full name"
+
+Forking the repository
+----------------------
+
+Firstly fork the repository by going to the `the Oniro Gitlab <https://gitlab.eclipse.org/eclipse/oniro-core/oniro>`_
+and clicking on the fork button that is in the top right of the screen.
+
+Working on your repository
+--------------------------
+
+Clone your fork by typing ``git clone`` and adding address of your fork which can be found by 
+clicking ``Clone`` button in your fork's website. You can clone it by SSH or HTTPS. Start 
+working on the project by creating a new branch. Make your changes, then add and commit 
+them. Each commit has to be signed off by the e-mail used in your Eclipse account 
+(add ``-s`` to every ``git commit``).Try to give relevant names to your commits, to point
+which file you add/modify. After that you can push the changes to your branch.
+
+
+Creating the Merge Request
+------------------------
+
+Go to your repository in an internet browser. Create a Merge Request by clicking ``Merge Requests`` on left toolbar
+and press ``New merge request``. Add an explainable description and create a merge request. Before merging, it has
+to be reviewed and approved by the |main_project_name| repository maintainers. Read their review and add any required
+changes to your merge request. After you polish your merge request the maintainers will run the pipelines which
+will check if your changes don't break the project and approve the project to merge it to the main project.
+Remember that changes which were added on top of changes introduced in the MR, should be squashed into the initial
+commit.