Merge pull request 'docs: add CONTRIBUTING.md' (#29) from def/add_contributing_guide into master

Reviewed-on: https://git.selfprivacy.org/SelfPrivacy/selfprivacy-rest-api/pulls/29

CONTRIBUTING.md

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+# Instructions for [VScode](https://code.visualstudio.com) or [VScodium](https://github.com/VSCodium/vscodium) under Unix-like platform.
+
+1. **To get started, create an account for yourself on the** [**SelfPrivacy Gitea**](https://git.selfprivacy.org/user/sign_up). Proceed to fork
+the [repository](https://git.selfprivacy.org/SelfPrivacy/selfprivacy-rest-api), and clone it on your local computer:
+
+    ```git clone https://git.selfprivacy.org/your_user_name/selfprivacy-rest-api```
+
+2. **Install Nix**
+
+    ```sh <(curl -L https://nixos.org/nix/install)```
+
+    For detailed installation information, please review and follow: [link](https://nixos.org/manual/nix/stable/installation/installing-binary.html#installing-a-binary-distribution).
+
+3. **Change directory to the cloned repository and start a nix shell:**
+
+    ```cd selfprivacy-rest-api && nix-shell```
+
+    Nix will install all of the necessary packages for development work, all further actions will take place only within nix-shell.
+
+4. **Install these plugins for VScode/VScodium**
+
+    Required: ```ms-python.python```, ```ms-python.vscode-pylance```
+
+    Optional, but highly recommended: ```ms-python.black-formatter```, ```bbenoist.Nix```, ```ryanluker.vscode-coverage-gutters```
+
+5. **Set the path to the python interpreter from the nix store.** To do this, execute the command:
+
+    ```whereis python```
+
+    Copy the path that starts with ```/nix/store/``` and ends with ```env/bin/python```
+
+    ```/nix/store/???-python3-3.9.??-env/bin/python```
+
+    Click on the python version selection in the lower right corner, and replace the path to the interpreter in the project with the one you copied from the terminal.
+
+6. **Congratulations :) Now you can develop new changes and test the project locally in a Nix environment.**
+
+
+## What do you need to know before starting development work?
+- RestAPI is no longer utilized, the project has moved to [GraphQL](https://graphql.org), however, the API functionality still works on Rest
+
+
+## What to do after making changes to the repository?
+
+**Run unit tests** using ```pytest .```
+Make sure that all tests pass successfully and the API works correctly. For convenience, you can use the built-in VScode interface.
+
+How to review the percentage of code coverage? Execute the command:
+
+```coverage run -m pytest && coverage xml && coverage report```
+
+Next, use the recommended extension ```ryanluker.vscode-coverage-gutters```, navigate to one of the test files, and click the "watch" button on the bottom panel of VScode.
+
+**Format (linting) code**, we use [black](https://pypi.org/project/black/) formatting, enter
+```black .``` to automatically format files, or use the recommended extension.
+
+**And please remember, we have adopted** [**commit naming convention**](https://www.conventionalcommits.org/en/v1.0.0/), follow the link for more information.
+
+## Helpful links!
+
+**SelfPrivacy Contributor chat :3**
+
+- [**Telegram:** @selfprivacy_dev](https://t.me/selfprivacy_dev)
+- [**Matrix:** #dev:selfprivacy.org](https://matrix.to/#/#dev:selfprivacy.org)
+
+**Helpful material to review:**
+
+- [GraphQL Query Language Documentation](https://graphql.org/)
+- [Documentation Strawberry - python library for working with GraphQL](https://strawberry.rocks/docs/)
+- [Nix Documentation](https://nixos.org/guides/ad-hoc-developer-environments.html)